“in”有“穿着、带着”的意思 ；one's 某人的 ；birthday 生日；suit 套装；字面意思是“穿着生日礼服”。但真实的意思却有另一种含义！birth 出生；birthday 出生那天；人出生时是什么都不穿，所以，in one's birthday suit是一丝不挂，赤身裸体，寸丝不挂，赤身露体的意思。

【例句】
I guess he didn’t know I was in the house. When I walked into the living room, he was just standing in his birthday suit. 
我猜他当时并不知道我在屋里。我走进起居室时，他正一丝不挂地站在那儿!

The little boy was having the sunbath on the beach in his birthday suit.
这个小男孩赤身裸体地躺在沙滩上晒太阳。