carelessness / negligence

油断: negligence

油断: to be negligent

being unprepared

(Classical Japanese, archaic) entrust, leave / (Classical Japanese, archaic) abandon oneself to / (Classical Japanese, archaic) devote oneself to

油断大敵: there's many a slip twixt cup and lip, carelessness is what will get you into trouble; security is the greatest enemy; Idleness is the devil's workshop

unpreparedness is the greatest enemy