adverb
secretly; privately; in confidence
Describes doing something discreetly, without others knowing; often used for private conversations or arrangements.
二人は密々に会って話し合った。
The two met secretly and talked.
その計画は密々に進められた。
The plan was carried out privately.
こっそり is a more common and casual adverb for doing something stealthily or on the sly. 密々 is more formal and emphasizes privacy rather than sneakiness.
Reduplication of 密 (みつ, 'secret, dense'), forming an adverb meaning 'secretly, privately'. The exact historical derivation is uncertain, but the reduplication intensifies the sense of secrecy.