The context is from the movie "Fear And Loathing In Las Vegas" The following text is the main character's narration of things and people he sees while walking around a casino in Las Vegas.
DUKE:
Who are these people? These faces!
Where do they come from? They look
like caricatures of used car
dealers from Dallas.
And, sweet Jesus, there are a hell
of a lot of them at four-thirty on
a Monday morning. Still **humping**
the American dream, that vision of
the big winner somehow emerging
from the last minute predawn chaos
of a stale Vegas casino.
The only definition I found that might have something to do with this context is this one 6. Slang. to exert (oneself) in a great effort. (It's from thefreedictionary.com)