The Collaborative International Dictionary of English v.0.48:

Inexpressibles \In`ex*press"i*bles\, n. pl.
   Breeches; trousers;; underwear generally, especially women's;
   -- now usually referred to as unmentionables. [Colloq. or
   Slang] --Ld. Lytton.
   [1913 Webster +PJC]