What do we have here? “Tree” (木) and “not” (不). Not a tree?!
This kanji has two yomi: HAI (the unvoiced form of the –pai in ippai) and sakazuki. Wow, that kun-yomi is lots of fun to say! Sakazuki! Sounds like “I like (suki) sake.” Probably has nothing to do with that, but fun to speculate!
Anyway, Henshall says that 木 here means “wooden item.” And 不 phonetically expresses “hold.” So 杯 is a wooden item for holding—a wooden vessel, which was once a wooden goblet. Now 杯 means “winecup” or “cupful.”
Coincidentally, I just ran into this kanji in a sentence by Mishima:
中尉は杯を干すと、それをすぐ麗子に与えた。
Chūi wa sakazuki o hosu to, sore o sugu Reiko ni ataeta.
As soon as the lieutenant had emptied his cup, he offered it to Reiko.中尉 (chūi: lieutenant) inside + company officer
干 (ho(su): to drink up, drain)
麗子 (Reiko: female name) of graceful beauty + childI feel sorry for any girl named Reiko who needs to learn to draw this first kanji! But then again, all Japanese kids will eventually learn it, as it’s the second character in 奇麗 (kirei: beautiful, clean, beautiful + of graceful beauty).
与 (ata(eru): to give, hand over)