na-adjective, noun
unbecoming; unsuitable; ill-matched
Describes something that does not suit a person, situation, or thing; often used for clothes, behavior, or pairings that feel mismatched or inappropriate.
彼女には派手な服は不似合いだ。
Flashy clothes are unbecoming on her.
その冗談はこの場に不似合いだった。
That joke was inappropriate for this occasion.
Formed from the negative prefix 不 (ふ) and 似合い (にあい), the noun form of 似合う (にあう, 'to suit'). The combination literally means 'not suiting'.