expression
one must be constantly on guard; one cannot be too careful
Used to warn that someone or something is so formidable or tricky that you must never let your guard down. Often followed by the person or thing requiring caution.
彼は油断も隙もない相手だ。
He is an opponent against whom you must never let your guard down.
この問題は油断も隙もない。
You cannot be too careful with this problem.
Compound of 油断 (carelessness) + も + 隙 (gap, opening) + も + ない (not). Literally 'there is neither carelessness nor opening', i.e., no room for inattention.