In your birthday suit
Definition 1 of 1
Pro Tip 1/3Informal Expression
This phrase is casual and might be inappropriate in formal contexts like job interviews or official documents. 
Don't say someone is in their birthday suit at a formal event; it's too casual.
Pro Tip 2/3Humorous Tone
The phrase often carries a light-hearted or humorous connotation when noting someone's nudity. 
At the beach, he joked about swimming in his birthday suit.
Pro Tip 3/3Not Literal
'Birthday suit' doesn't mean an actual suit; it's a playful way to say naked. 
Imagine forgetting your swimsuit and having to go in your birthday suit!