凹む
へこむ · hekomu · JLPT N2 · verb-godan
to be indented
🥫 凹 = concave, the opposite of 凸 convex; 凹む = to go concave, pressed inward
The kanji 凹 and its mirror 凸 form a pair: 凸 bulges out, 凹 sinks in. The okurigana む makes it a verb, to sink in or get dented.
凹 — pictograph of a concave shape
凹 is a pictograph depicting a concave or sunken shape, with its outline literally dipping inward in the middle. It is one of a famous pair with 凸 (convex), and the character itself visually shows the meaning 'concave, hollow, dented.' It has no phonetic component — the form is the meaning.