Meaning of idiom 'in one's birthday suit'

[Mirko]

English Idioms and Expressions, Advanced Level

ESL/EFL Test #46 "Clothing Idioms (2)", question 8

After all these years, Teresa still hasn't seen her husband in his birthday suit.

(a) in a three piece suit
(b) in his good Sunday suit
(c) naked

English Idioms and Expressions, Advanced Level

ESL/EFL Test #46 "Clothing Idioms (2)", answer 8

After all these years, Teresa still hasn't seen her husband naked.

Correct answer: (c) naked
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i could not understand it.

Mirko

[Mister Micawber]

The idiom 'in one's birthday suit' means 'unclothed; naked'.
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Native English teacher at Mister Micawber's