in one's birthday suit

ADJECTIVE
22 LETTERS · STARTS WITH I

in one's birthday suit is a valid 22-letter word worth 27 points in Scrabble and 29 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
I1N1O1N1E1'0S1B3I1R1T1H4D2A1Y4S1U1I1T1
1 + 1 + 0 + 1 + 1 + 1 + 0 + 1 + 0 + 3 + 1 + 1 + 1 + 4 + 2 + 1 + 4 + 0 + 1 + 1 + 1 + 1 =27 PTS
DOUBLE WORD → 54TRIPLE WORD → 81
MEANING

What does in one's birthday suit mean?