expression, noun
one's best interests; one's own good
Used when an action or advice is for the person's own benefit or well-being. Often appears in phrases like 身のためにならない or 身のためを思って.
そんなことをするのは身のためにならないよ。
Doing that kind of thing isn't in your best interests.
彼は身のためを思って忠告した。
He gave advice out of concern for his own good.
ため alone means 'for the sake of' or 'benefit'; 身のため specifically refers to one's own personal benefit or well-being.
Compound of 身 (み, 'body, self') + の (possessive particle) + ため ('sake, benefit'). The exact historical derivation is straightforward; the phrase has been used to express personal benefit or interest.