似合う
にあう · niau · JLPT N3 · verb-godan
to match
👗 似合う = 似 resemble + 合う fit: it fits you like it was made to match
似 means to resemble and 合う means to fit or match. Something that 似合う you fits and matches the kind of person you are. Use it with が: 赤がよく似合う means red really suits you.
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.