A diaper is a cloth that is used to wrap a baby. The diaper is changed when the baby makes the diaper wet or the baby defacates. If somebody says that somebody else diapered them, it can only be three possiblilities. - A story is being told from the fictional viewpoint of the baby. - A grown person is helpless, cannot get to the toilet and must have diapers. - It is a metaphor for treating someone like a baby. "If you can't behave like an adult, then I'll just have to come around every day to change your diapers."

An adult might also say this to emphasize a fundamental, longstanding relationship. When the adult was a baby, the adult's diapers had been changed by the mother or some other caregiver.