1
general::
Phrase(s): on duty
at work; currently doing one’s work. (The opposite of off duty.) • I can’t help you now, but I’ll be on duty in about an hour. • Who is on duty here? I need some help.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
At one's post, at work, as in The new nurse was on duty that evening, or The watchman was fired because he was drunk on duty. [Mid-1600s] The antonym, off duty, means "not engaged in one's work," as in Captain Smith was much more amiable when he was off duty. [Mid-1800s]
American Heritage Idioms