taru-adjective, adverb taking the 'to' particle
openly in broad daylight; brazenly in public
Describes an action done without shame or concealment during the day, often with a sense of audacity. Used adverbially with と or as a たる adjective.
犯人は白昼公然と銀行に押し入った。
The criminal brazenly broke into the bank in broad daylight.
白昼公然たる不正が行われていた。
Blatant wrongdoing was being carried out in broad daylight.
A yojijukugo combining 白昼 (broad daylight) and 公然 (open, public). The exact historical derivation is uncertain, but the compound is used to emphasize shamelessness in public during the day.