noun
dandy; dude; fashionable man
Refers to a man who is stylish, well-dressed, and often somewhat showy or flamboyant. Can carry a slightly old-fashioned or literary flavor.
彼はいつも洒落た服を着ている伊達者だ。
He's always a dandy, wearing stylish clothes.
あの伊達者は帽子のセンスがいい。
That dude has a good sense of hats.
伊達 (date) refers to the Date clan, known for flamboyant style; 者 (sha/mono) means 'person'. The exact historical derivation is uncertain, but the term is conventionally associated with stylishness.