noun, noun which may take the genitive case particle 'no'
leaving to chance; drifting; happy-go-lucky
Describes an attitude or situation where one does not actively control the outcome, but lets events take their own course. Often used with 〜の or 〜にする.
See also: 成り行きに任せる (なりゆきにまかせる)
彼は成り行き任せの人生を送っている。
He lives a life of drifting, letting things take their own course.
計画を立てずに、成り行き任せで旅行することにした。
We decided to travel without a plan, just going with the flow.
Compound of 成り行き (course of events) + 任せ (leaving to). The exact historical derivation is uncertain, but the meaning is transparent from its parts.