birthday suit

wear one's birthday suit (wearing nothing but skin; the state of nakedness) — в чем мать родила; нагишом

Example: Jane showed up at the party wearing nothing but her birthday suit.

not have a stitch 'on AND not be wearing a 'stitch (to be naked) {infml} — голый, без ничего (об одежде)

see also
[as naked as sb's mother bore him]