on duty; while on official business
Used for public officials, civil servants, or employees when they are engaged in official duties. Often appears in formal contexts or news reports.
A police officer on duty was involved in an accident.
Please refrain from personal calls while on official duty.
Compound of 公務 (こうむ, 'official duties') + 中 (ちゅう, 'in the middle of'). The reading is standard on'yomi.