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.
He is an opponent against whom you must never let your guard down.
You cannot be too careful with this problem.
Standard kanji spelling.
Variant using 無い for ない.
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.