Not have a stitch on
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
The phrase is informal and best used in casual or humorous contexts, not in formal situations. 
At the beach party, Mike joked that he didn't have a stitch on under his towel.
Pro Tip 2/3Privacy Expectation
The phrase often implies surprise or embarrassment when someone is seen naked unexpectedly. 
When Lisa walked into the room, she realized she didn't have a stitch on and quickly covered up.
Pro Tip 3/3Humorous Tone
It's often used humorously to describe someone's nudity or to make light of an awkward situation. 
He lost his towel and had to run back inside without a stitch on, making everyone laugh.