This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means hard so the verb vocab version? To make something hard. You need to differentiate this from 固まる, though, which means "to harden" (the hardening happens on its own). This word means "to make something hard," because you're doing the hardening. You can remember this because you want to make something hard. And to do that, you send special "mail" (める) to this thing you want to make hard, and this mail does that for you.