合う
あう · au · JLPT N4 · verb-godan
to fit
🧩 合 = lid + mouth: a lid that fits its pot, so things match
The kanji 合 shows a cover (亼) over an opening (口): a lid closing neatly on a container. From that comes the sense of fitting and matching. Sound hook: au is like the ow you say when a shoe does not fit.
合 — 乛 (lid/roof) + 口 (mouth)
Originally a pictograph of a lidded vessel — a cover (亼) closing over an opening (口) — giving the core sense of 'come together, join, fit.' From 'fit/match' it extended to 'combine, unite, agree' and to measure-word uses. The reading hé reflects this 'joining' meaning.