The Collaborative International Dictionary of English v.0.48:

Mother-naked \Moth"er-na`ked\, a.
   Naked as when born; completely naked.

   Syn: naked as the day one was born, in one's birthday suit.
        [1913 Webster]

WordNet (r) 3.0 (2006):

mother-naked
    adj 1: as naked as at birth [syn: mother-naked, naked as the
           day one was born, naked as the day you were born, in
           one's birthday suit, in your birthday suit]