凹
āo , wā · HSK 7
concave
🕳️ āo sounds like ow — what you shout when you step in a dent
The character itself has a sunken middle, its strokes literally form a concave shape. The alternate reading wā echoes whoa, as if stumbling into a hollow.
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.