adverb
alertly; vigilantly; without letting one's guard down
Adverbial form of 油断 (carelessness, negligence) with なく, meaning 'without being careless'. Used to describe acting with constant attention and caution.
彼は油断なく周囲を見張っていた。
He was vigilantly watching his surroundings.
油断なく準備を進めることが大切だ。
It is important to proceed with preparations without letting your guard down.
Derived from the noun 油断 (carelessness) + なく (adverbial form of ない, 'not'). The word 油断 itself is said to originate from a Buddhist story about a monk who spilled oil, but the exact derivation is uncertain.