Oh my god. I just realized that carboxyl is a portmanteau of carbonyl and hydroxyl.
I feel so dense for not realizing this earlier! It was staring me in the face the entire time we’ve been learning about functional groups in Honors Bio. But this time, as I was looking at the diagrams, I thought, “Oh, hey, the carboxyl group (COOH) looks like it’s made up of a carbonyl (CO) and a hydroxyl (OH). Wait a minute…”