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.
The criminal brazenly broke into the bank in broad daylight.
Blatant wrongdoing was being carried out in broad daylight.
白昼 means 'broad daylight' or 'midday' and is a noun; 白昼公然 adds the nuance of openness and shamelessness.
公然 means 'openly' or 'publicly' and can be used in various contexts; 白昼公然 specifically emphasizes the brazenness of doing something in 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.