slang : great in quantity or amount : many, much
I cannot say I've seen this word used out of the French context. According to M-W the word is playful slang, although I'm not sure who casually tosses around the word BEAUCOUP. I would consider a slang synonym of "many" to be something more like "a shitload."* But that's just me.
*See also: buttload, assload, fuckload.
Here is a picture of BEAUCOUP rats.