adverb
without one's knowledge; easily
Rare literary adverb meaning doing something without the person concerned knowing, or doing something effortlessly. Often used in the pattern 〜に.
彼は闇々にその計画を進めた。
He proceeded with the plan without anyone's knowledge.
彼女は闇々に難しい問題を解いた。
She solved the difficult problem with ease.
やすやす is the common adverb for doing something easily; 闇々 is rare and old-fashioned.
Reduplication of 闇 (やみ, darkness), literally 'dark-dark', implying something done in the dark or without notice. The exact historical derivation is uncertain.