It is obvious that "beaucoup" is formed from 2 words: beau "fine, great" and coup "stroke", but what's not so obvious (at least at first glance) is how/why it's evolved to mean "many, much".
I found the following explanation of beaucoup's etymology on www.etymonline.com:
French, literally "a great heap," from beau "fine, great" (see beau (n.)) + coup "a stroke," also "a throw," hence, "a heap" (see coup (n.)). Cf. Spanish golpe "multitude."
I see some logic here but maybe there are more detailed story / other explanations available?