without a stitch on
Completely nude; having no articles of clothing on whatsoever. As I walked into the kitchen, I saw my roommate by the counter without a stitch on. I nearly dropped my mug of tea out of shock! When you were a toddler, we used to let you run around the beach without a stitch on.
without a stitch on
Naked, as in They let their baby run around outside without a stitch on. A related phrase is not have a stitch on. These expressions use stitch in the sense of "a piece of clothing," a usage dating from the early 1800s.