expression, i-adjective
treacherous; dangerous; untrustworthy; sneaky; cunning
Describes a person or situation that cannot be taken lightly; someone who is sly, unpredictable, or likely to cause trouble if you let your guard down. Often used in the phrase 油断ならない相手 (a person you can't afford to be careless with).
彼は油断ならない相手だ。
He is a dangerous opponent you can't let your guard down around.
この道は夜になる
This road is treacherous at night.