My two are: (1). the infamous travelers’ complaint “Dire Rear” (2). A man known (and feared) as “Grevious Bodily” Harmon.
It’s not just a Terry-ism, but that’s where I got it - I now say all the time “Pull the other one, it’s got bells on.”
Living in the States as I do I get a lot of baffled looks.
Knurd, the opposite of being drunk.
Splatters, the troll version of bouncers. Trolls use more force.
“Multiple exclamation marks are a sure sign of a diseased mind.”
My absolute favorite isn’t from the Disc, but it involves a Universal Character:
*- “You’re Hells Angels, then? What chapter are you from?”
- REVELATIONS, CHAPTER SIX.*
Otherwise, it has to be Nanny either singing the Hedgehog Song, or commenting on Granny about to land on someone like a ton of bricks.
Is that from Good Omens, silenus? It sounds very familiar.
Getting an education is a little like getting a communicable sexual disease. It makes you unsuitable for a lot of jobs and gives you the urge to pass it on to others.
Yep, Good Omens.
On the predictive powers of tarot cards (and similar):
“Cardboard isn’t very bright.”
The three rules of the Librarians of Time and Space are: 1) Silence; 2) Books must be returned no later than the date last shown; and 3) Do not interfere with the nature of causality.
(from Guards! Guards!)
From Maskerade, referring to Granny Weatherwax:
“People who didn’t need people needed people around to know that they were the kind of people who didn’t need people.”
There are so many, but two I predominately use at work when I have to kick people off property:
“THERE IS NO JUSTICE, THERE IS JUST US”
and
“Have you not heard of Rule One?!”
The Patrician believed in Democracy - ‘one man, one vote’.
The Patrician was that man and he had that vote.
I believe it’s :
“Ankh-Morpork had dallied with many forms of government and had ended up with that form of democracy known as One Man, One Vote. The Patrician was the Man; he had the Vote.”
Eldritch, which means oblong.
“He lit a match, the darkness blossomed into a ball of sulphurous white light, and the Librarian dropped on him like the descent of Man.”
–Sconner unwisely tries to set fire to the Library of Unseen University, Sourcery.
Oook.
(It’s all in the pronunciation.)
“The Yen Buddhists are the richest religious sect in the universe. They hold that the accumulation of money is a great evil and burden to the soul. They therefore, regardless of personal hazard, see it as their unpleasant duty to acquire as much as possible in order to reduce the risk to innocent people.”
There is a neverending supply of these, but here’s a few of my favorites 
There was not a lot that could be done to make Morpork a worse place. A direct hit by a meteorite, for example, would count as gentrification.
– (Terry Pratchett, Pyramids)
FABRICATI DIEM, PVNC.
– The motto of the Ankh-Morpork City Watch (Terry Pratchett, Guards! Guards!)
They say the heat and the flies here can drive a man insane. But you don’t have to believe that, and nor does that bright mauve elephant that just cycled past.
– (Terry Pratchett, The Last Continent)
“Give a man a fire and you’ll warm him for a night. Set a man on fire and you’ll warm him for the rest of his life.”
“Just because someone’s a member of an ethnic minority doesn’t mean they’re not a nasty small-minded little jerk.” (FoC)
There was a roar like the scream of a camel who has just seen two bricks. (SM)