na-adjective, noun
self-conceit; overconfidence; neglecting one's health
Describes a person who is overly confident or reckless, often ignoring their own limitations or well-being. The 'neglecting one's health' nuance is less common and typically appears in contexts of pushing oneself too hard.
彼は身知らずにも、一人でその大仕事を引き受けた。
He was so overconfident that he took on that huge task all by himself.
身知らずの行動で、結局体を壊
Through reckless behavior, he ended up ruining his health.
Compound of 身 (mi, 'body, oneself') and 知らず (shirazu, 'not knowing'). The exact historical derivation is uncertain, but it literally suggests 'not knowing oneself' or 'not knowing one's own limits'.