Millions,
Possums ride piggyback. Toddlers ride piggyback. Do pigs? I don’t think so. Maybe, being a city boy, I’ve missed the nuances of piglet playtime or courting rituals, but, since they don’t have arms or legs long enough to pull this off anyway, the best a pig could do would be to lie aimlessly on another pig’s back. So, two questions:
Try here for some insight.
perhaps, but this may be an alternative explanation http://www.lifesaboar.co.nz/photos/pig-hunting-15.html
Weird that the random house link also contained an apology for “being a city boy.” I feel so dirty.
It’s an example of “folk etymology” – where a familiar word is substituted for an unfamilar one. “Pick back” sounded odd, so “piggyback” was coined.
Any other good examples of this? I recall hearing about a few from similar threads, strange linguistic students, and the like, but I’m blanking.
When tom-tom stole that pig he grabbed it by the legs , swung it up on his back
and ran away.
When he got home his 'lil’namesake cried to be given a ride like he’d just given the pig!
ERGO-----nany a father’s aching back!
ez
Ah, cool.
Toddlers ride horsieback here. Makes more sense, right? 
While we’re at it: Why is it called a PiggybaNk?
We don’t put money in real pigs, do we? [well…I don’t]
I actually read something about that; IIRC, the clay used to make banks, back in the “olden days” (my catch phrase for being bad at date memorization) was called “pihg” or “pigh” or some similar sounding word in a foreign dialect. Traders brought these to English speaking countries, where it was bastardized (or “folk etymolog…ized”) into piggybank. Eventually, this small change resulted in people actually firing clay banks that did in fact look like piggies.
Ok, I was a little off. Wikipedia has this to say.
Or, just read this, which is the pertinent section (quoting from above link):
"In Middle English, “pygg” referred to a type of clay used for making various household objects such as jars. People often saved money in kitchen pots and jars made of pygg, called “pygg jars”. By the 18th Century, the spelling of “pygg” had changed and the term “pygg jar” had evolved to “pig bank.” This name may have caught on because the pig banks were mostly used by children, and the pig is a child-friendly shape that is easy to fashion out of clay. The actual origin of the name bears no relation to the pig itself.
What also helped to start the change from jar to pig was during the 18th century, an order of 200 “pygg” jars was sent for by a shop owner. The person on the receiving end of this letter did not understand and hand crafted 200 coin holders that were shaped like pigs. When this order was received they were shelved and sold very quickly. The new pig shaped coin holder became popular. Once the shop had ran out of stock, they sent off for more pig-shaped coin holders."
Two classic examples I remember from my few linguistics classes were hangnail and bridegroom.
Hey, Thanks, Birdmonster. A type of clay, huh.
Good thing they didn’t use palygorskite jars.
Doesn’t sound too well, right?
“Put that dollar auntie Em gave you in your palygorskite bank, dear”
Nope.
Well, if you’d grown up on a farm, and seen a boar (male pig) and a sow (female pig) in the act of conceiving litters of little piglets, you would have seen a pig do that. Or pretty close to it, anyway.
But I think the other explanations are more likely for the actual derivation of this word.
Bridegroom isn’t an error though. The word “shifted” from bride man to bride man. And the word grom or guma became groom, still meaning man, at the same time. So basically the dual-words just changed in sync with their main spellings.
A similar “problem” is sometimes given that “helpmate” originally meant “helpmeet”. Which might be interesting except that meet was just the old spelling of mate, not a different word.