adverb
diligently; faithfully; without fail
Used to describe doing something carefully and consistently, without neglecting duties or making careless mistakes. Often implies a sense of responsibility and thoroughness.
彼は怠りなく仕事をこなす。
He handles his work diligently.
毎日怠りなく日記をつけている。
I keep a diary every day without fail.
怠る is the verb meaning 'to neglect' or 'to be lazy about'; 怠りなく is the adverbial form of its negative, meaning 'without neglecting'.
真面目に means 'seriously' or 'earnestly', focusing on attitude; 怠りなく emphasizes consistent, careful action without lapses.
Adverbial form of the negative of the verb 怠る (おこたる, 'to neglect'), literally meaning 'without neglecting'.