Definition & Meaning of "in {one's} birthday [suit]"

What is the origin of the idiom "in one's birthday suit" and when to use it?

The origin of "in one's birthday suit" is believed to stem from the idea that a person is born without clothing and, therefore, their "birthday suit" is their natural state of being. It is used to describe someone who is completely naked, without any clothing. It is a playful and humorous phrase that emphasizes the absence of garments.