Meaning of "never cop a quart"

Question

What does "never cop a quart" mean? It's from the movie Nightmare Alley.

Clem : Folks know to come to me for it.

Half a plug a quart.

Never cop a quart.

You gotta make the tin cling, same as everybody else.

Stan: I never touch the stuff.

Clem: Ain’t we virtuous?

Just say you understand.

Stan: I understand.

Clem: What do you understand?

Stan: Never cop a quart.

Clem: That’s right.

Toe the line, don’t fuck with me.

That’s the lesson.

Answer 1

It's "carny" (US carnival and fairground worker) slang, not a common idiom.

"Cop" means "take, (especially by deception)". Clem is saying that Stan has to pay for alcohol, just like everyone else. The price is "Half a plug". I can't find a reference to how much that is, but I'd guess "a plug" is a dollar. It would make a quart worth about $10 in modern money, which seems about right (for probably untaxed and illegal alcohol). "Make the tin cling" means to put coins in the moneybox.

It's not standard slang. Don't learn this expression to use it.

Answer 2

While I think that James K is quite likely correct that in this particular case, this is meant to be "carny" slag, "cop" meaning "take" or "steal" has wider currency than that.

Merriam-Webster gives one sense of "cop" as:

Definition of cop (Entry 2 of 4)

transitive verb

1. slang : to get hold of : catch, capture also : purchase
2. slang : steal, swipe
3. : ADOPT sense 2 cop an attitude

This indicates that these senses are slang, but not specifically carny slang.

Dictionary.com gives:

A cop is an informal term for a police officer. As a verb, cop is used in a variety of slang expressions meaning “grab” or “obtain,” from copping a feel on someone (not recommended) to copping out on going to a party (= not going) to copping to (confessing to) eating the last slice of pizza.

The many, seemingly unrelated, meanings of cop start to make sense when you know where the word comes from. Via French, cop ultimately comes the Latin capere, or “to seize, snatch, take, grab.”

Cop became slang for “seizing” in the early 1700s. This verb may have given rise to copper, thieves’ slang for “law enforcement” by the 1840s and shortened to cop by the 1850s. For much of its history, it was often seen as dismissive or derogatory, though most police officers are just fine with it in contemporary use.

Other theories root cop as an acronym for constable on patrol (unlikely) or as a reference to copper badges early policemen wore in New York (probably influenced the term).

Cop has taken on verb many other senses in the 20th century. We can find cop a feel, or “to grope someone,” in the 1930s as well as to cop to something, or “confess.” We can find cop on, or “understand something,” in the 1940s, the same decade there’s evidence for cop out, or “give up.” In the 1950s, we can find to cop as in to obtain illegal drugs.” We can find cop an attitude, or “assume an adamant stance,” in the 1970s.

Collins gives:

in American English
verb transitive
Word forms: copped or ˈcopping

Slang

1. to seize, capture, win, steal, etc.
2. to buy (drugs)

So the word "cop" as a verb has a sense of "grab", "take" or "steal" in informal use goign back hundreds of years, and is in no way limited to use by carnies.

I think that most of these senses of "cop" are now somewhat out of fashion, but "cop a feel" meaning "grope someone" or "fondle someone sexually without permission or warning" is current. They are all slang, however.