In French, as you may know, beaucoup is an adverb meaning “a lot” or “much” (as in merci beaucoup, meaning “thanks a lot.”) Beaucoup isn’t used on its own as an adjective in French; if you want to say “many” in French, you would use the phrase beaucoup de. In other words, you would say beaucoup de livres (“a lot of books”), not beaucoup livres. But French grammar was thrown to the wind when English speakers borrowed this word. Beaucoup has been used as a playful slang adjective in English since at least 1918.