taru-adjective, adverb taking the 'to' particle
openly; publicly; in broad daylight
Rare literary term describing an action done without any attempt at concealment, often with a negative or brazen connotation. Used adverbially with と.
彼らは公々然と賄賂を受け取っていた。
They were openly accepting bribes.
公々然と嘘をつくその態度に呆れた。
I was appalled at his attitude of telling lies so brazenly.
公然 is the common modern word for 'openly' or 'publicly'. 公々然 is a rare, more literary variant with a similar meaning but stronger emphasis on brazenness.
Formed from the kanji 公 (public) reduplicated with the suffix 然 (zen, 'in the manner of'). The exact historical derivation is uncertain; it is a rare literary coinage.