taru-adjective, adverb taking the 'to' particle
quietly and assiduously
Describes working steadily and silently, often with dedication. A literary, somewhat formal expression; used adverbially with と or as a たる adjective.
彼は営々黙々と研究を続けた。
He continued his research quietly and assiduously.
営々黙々たる努力が実を結んだ。
Their quiet, assiduous efforts bore fruit.
Means 'silently' or 'without a word'; 営々黙々 adds the nuance of steady, diligent work.
Means 'assiduously' or 'industriously'; 営々黙々 combines this with quietness, while 孜々 focuses purely on diligence.
Compound of 営々 (diligently, from reduplication of 営 'work') and 黙々 (silently, from reduplication of 黙 'silent'). The exact historical derivation is uncertain, but the combination is a natural pairing of two reduplicated kanji compounds.