Jack: My son is such a trouble maker.
Martha: Tell me about it. My little Billy escaped from his bath yesterday and ran around the neighborhood in his birthday suit!
When a baby is born, it is not wearing any clothes. So your “birthday suit” refers to the clothes you were wearing when you were born: nothing!
ジャック：うちの息子は、本当にトラブルばかりを起こすんだ。
マーサ：気持ちはわかるわ。うちのビリーも昨日お風呂から逃げ出して、近所を素っ裸で走り回ったのよ。
赤ちゃんは、生まれるときには服を着ていないですよね。ですので、「誕生日スーツ」は生まれたときに着ていた服のことです…つまり裸！
