油断大敵(ゆだんたいてき)

A well-known 四字熟語(よじじゅくご) (four-character idiom) meaning "carelessness is one's greatest enemy."

Etymology

• 油断(ゆだん): carelessness, letting one's guard down
• 大敵(たいてき): great enemy, formidable foe

Usage

• Used as a standalone warning: 油断大敵(ゆだんたいてき)だ (Carelessness is dangerous!)
• Common in sports, exams, work, and safety contexts
• Often used to motivate continued effort even when things seem to be going well
• Frequently appears on posters and signs in workplaces

Similar words

• 油断(ゆだん)するな: Don't let your guard down
• 気(き)を抜(ぬ)くな: Don't lose focus
• 油断(ゆだん)は禁物(きんもつ): Carelessness is forbidden

Equivalent English proverbs: "Don't count your chickens before they hatch," "Pride goes before a fall."