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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.