do (one's) duty
1. To do, undertake, or accomplish that which is expected or required from one. I know it isn't easy facing off against the lowlifes of this town, but you're on the police force and you have to do your duty.
2. To urinate or defecate, especially a house pet. I hate standing out here in the cold, waiting for the dog to do his duty.
do one's duty
1. Do one's tasks or what is expected of one. For example, He was only doing his duty when he made the children finish their work. [Early 1500s]
2. Defecate or urinate, as in Please keep the dog outdoors until he does his duty. This euphemism for attending to such needs is heard less often today. [1930s]