be on your guard!; take care!
Proverb warning against letting one's guard down; literally 'carelessness is forbidden'. Used as a standalone admonition or within a sentence.
You mustn't let your guard down. Let's check once more.
Even after the exam is over, you must stay vigilant.
A synonymous proverb with a stronger, more emphatic tone: 'carelessness is a great enemy'. 油断は禁物 is slightly more common in everyday cautionary advice.
A set phrase combining 油断 (carelessness, negligence) with 禁物 (something forbidden, taboo). The exact origin of 油断 is uncertain, but it is often explained through a folk etymology involving the difficulty of pouring oil without spilling.