na-adjective, noun
as one pleases; at one's own discretion; selfish
Describes doing things entirely according to one's own wishes or convenience, often with a negative connotation of being self-centered or arbitrary. Used both as a noun and as a na-adjective.
彼はいつも勝手次第に行動する。
He always acts as he pleases.
そんな勝手
Such a selfish argument won't be accepted.