be on duty
fixed phrase.fixed phrase. to be at work and responsible for your job at a specific time.
fixed phrase. to be engaged in one's professional obligations or assigned tasks during a scheduled period. This phrase functions as a predicative adjective phrase following the copula 'be'.
The nurse will be on duty until eight tonight.
I cannot join you for lunch because I will be on duty at the security desk all afternoon.
The commanding officer required at least two senior guards to be on duty throughout the duration of the diplomatic summit.
usually refers to jobs that involve shifts or public service, such as police, medical staff, or soldiers.
contrast with 'off duty' to show how the prepositional change flips the meaning; you can also mention 'active duty' for military contexts.
The doctor is in duty today.The doctor is on duty today.the fixed expression requires the preposition 'on' rather than 'in'.
Last updated July 26, 2026