be on your guard!; take care!
A set phrase warning against carelessness; often used as an admonition or reminder to stay vigilant. Equivalent to 油断は禁物.
See also: 油断は禁物 (ゆだんはきんもつ)
Be on your guard! Don't let your attention slip until the very end.
During the exam, you must stay vigilant.
The full form with the particle は; identical in meaning but slightly more explicit. 油断禁物 is a compact version often used in admonitions.
A compact form of the proverb 油断は禁物 (carelessness is forbidden). The exact historical derivation is uncertain, but it is a well-established set phrase.