Which doesn’t mean trousers. In fact, it means trousers are LESS likely, since, if anything, trousers are likely to be a concession to muggle students, and muggle-influenced fashions among the youths, given that wizard’s robes - likely meant to be worn as the teachers do, rather than the way the students are often shown - are the traditional garments.
Snape is all about Slytherin, which is the house least friendly to Muggle fashions and most friendly to traditional wizard fashions. That would argue for the likelihood of him going “au naturel” under his robes.
Yeah, this is the best argument I’ve seen so far for Snape’s behavior. Even though he was a half-blood and was exposed to Muggle males wearing trousers, I guess it made sense (especially since his hated father was a Muggle) that he’d want to get as far away from Muggle-influenced styles as possible.
Though as someone pointed out someplace else I was reading, there was a “fad” going around during that period (Lupin mentioned it when Harry asked him about the event later on) where you couldn’t walk down a hall without getting hoisted up by your ankles. I would think that even the most hardcore wizard purists would at least make sure they were wearing clean boxers. 
I’m sure they were clean, they were gray–which means dingy, ie, his house elf or someone was either washing them with colors or not using bluing or bleach or both.
Bigger question: if the house elves obtain freedom by getting clothes from wizards, who does all that laundry?
Technically, don’t the clothes have to be given to the house elf, as contrasted with handing one’s clothes to the house elf with no transfer of ownership? The ploy Harry uses to free Dobby has always seemed a bit suspect. Alternately, one could direct the house elf to wash one’s dainties, left over there in the hamper, without handing them to the elf.
(“Master has given Dobby his tighty whiteys! Dobby is free!”)
Mmmmm, pants-less Snape…
What? A plothole? Pshaw! I’m busy over here, dammit!
it seems there is a tradional school uniform. white shirt, tie, jumper, trousers, skirt, with the robes worn over like a blazer. at least in the movies.
perhaps in the seventies the uniform was a bit different. or snape only had one or two pair of trousers and they were in the laundry?
a quick reread of the scene, she doesn’t mention clothing other than robes or the unfortunate unmentionables.
That doesn’t mean the trousers weren’t there. She doesn’t mention them breathing in and out of digesting or sweating, either, but we can assume they were doing a bit of all 3…
Tighty-whiteys? You don’t deserve a house elf–elves prefer masters who wear boxers… 
But I’m female, so it would be deliciously perverse.
I remember Dobby, in the book version of CoS, telling Harry that the Malfoys are careful not to pass him so much as a sock (or something) because then Dobby would be free to leave their service. And in OotP, Dobby ends up cleaning Gryffindor tower all by his lonesome because the other elves are insulted by all the hats, scarves, and socks Hermione hides all over the place in an attempt to free them.
But on the other hand, Kreacher has no problem taking Harry’s cloak and hanging it up for him in Deathly Hallows.
So who knows? I always assumed wizards made other arrangements for their laundry. Do you really need a washing machine if you’re a wizard? You can just point your wand and say “Allus Temperus Cheerus!” and bam! Your clothes are clean.
I never thought too much about what wizards wore under their robes in the books; Harry’s equipment list in the first book only lists “robes” as required uniform, but it seems like one good updraft should have convinced the powers-that-be long ago that some kind of trousers would be useful.
I think the trousers only came in for the movies. I never got the impression that they were worn under robes at any time. In fact it seemed to me to be a glaring departure in the movies. The list for school only mentioned robes, not sweaters or anything else like that. The Weasley kids might have had jeans or such, since their mother sent them sweaters for Christmas, but there’s not mention of any other clothing in any of the books, really, and a lot of the kids who came from wizarding families might not have had any experience with the muggle world, so they wouldn’t have any muggle clothes. It’s always robes, even in the off hours.
From The Harry Potter Lexicon:
Okay, from Mary GrandPre’s illustrations:
Here’s a Quidditch outfit, showing pants.
Here’s Ron burping slugs while wearing Muggle garb.
Here’s Harry fighting a boggart while wearing Muggle garb.
Here’s Harry fighting a dementor, with robe and pants clearly evident.
Near the kitchen, wearing robes that brush the ground–no pants evident.
Wormtail wearing robes and pants.
The Order of the Phoenix appears to wear robes and pants, though this is not entirely clear at this resolution.
Umbridge wears robes over a dress.
But Dumbledore and Voldemort, with Dumbledore wearing robes over a robe.
Slughorn attired in–well, I’m not sure quite what.
Draco using a mirror that magically erases the lower half of the body in order to preserve the pants mystery.
Draco and Filch, both with pants.
The marauders appear to wear open robes, suggesting the existence of pants.
Note that Harry’s “robe” appears sometimes to be a cape instead. Note that Snape, Voldemort, and Bellatrix (not linked) wear a different, high-collared style of robe.
I think the clothes thing only applies to masters of the house elves; otherwise Harry could have given the sock to Dobby himself. I suppose Hermione counts as a mistress of the house elves.
But wait - did Harry inherit Kreacher from Sirius? If so, I’m stumped.
Yes,Harry inherited Kreacher from Sirius.
You guys must have been very sad to watch Dogma then. Poor Alan was genderless.
Never saw it, problem solved.
I don’t really have an answer to the OP. Perhaps he just didn’t have money for pants? I mean, if you’re that poor, maybe that’s all you have. But I like Tengu’s explanation best.
Yeah, that was unexpected. But I can’t be sad when he’s on screen.
Oh, god, if I want to watch Dogma, I have to put up with Ben Affliction.
Although reading it on Wikipedia, it seems a little…farfetched. To say the least.
But Umbridge wears that nasty pink cardigan OVER her robes (she was clueless in so many ways, no?)–otherwise, the students couldn’t have commented on it because they wouldn’t have seen it.
I always assumed they had regular clothes on under the robes–Hogwarts is located in the north of England (Scotland?), supposedly–these kids would die of exposure if they only wore robes.
Or perhaps it’s personal preference: there is that one wizard who wears that nighty which is robe like at the World Cup and says he likes a breeze around his privates…
I like the school uniforms–although they do change over the films. In HP1, their school ties have the school insignia or herald/shield (whatever) on them–the other movies, each house has its colors as the tie colors. There are other changes, in the stripes on the sweaters (sorry, jumpers) etc. I’m glad they dropped the “dunce cap” look from the first film. YMMV.
PS-I’ll take Alan Rickman with robes, without robes, in or out of trousers–any way I can…