i-adjective
decent; respectable; presentable
Often used in negative constructions like みっともいいものじゃない to mean 'not decent' or 'not respectable'. The antonym みっともない is far more common.
Antonyms: みっともない
そんな格好で出かけるのはみっともいいものじゃない。
Going out dressed like that is not decent.
彼の振る舞いは決してみっともいいとは言えなかった。
His behavior was by no means respectable.
Derived from 見っとも (mittomo), an emphatic form of 見る (miru, 'to see'), combined with いい (ii, 'good'). The exact derivation is uncertain, but it likely originated as a colloquial positive counterpart to みっともない.