Inexpressibles

 
Inexpressibles - During the Victorian era it was not considered polite for a lady to describe gentlemen as wearing breeches so they were called inexpressibles. Although the word was occasionally used for women's underwear it was far more frequently used for men's outer legwear - breeches or trousers.

By contrast the word more likely to be used nowadays - unmentionables - is far more usually appled to ladies' underwear.

After this, there was a good deal of dodging about, and hitching up of the inexpressibles in the absence of braces, and then the short sailor (who was the moral character evidently, for he always had the best of it) made a violent demonstration and closed with the tall sailor, who, after a few unavailing struggles, went down, and expired in great torture as the short sailor put his foot upon his breast, and bored a hole in him through and through.
The Life and Adventures of Nicholas Nickleby by Dickens, Charles

I had barely the time, as he made for the cabin door, to grab him by the seat of his inexpressibles.
Falk - A Reminiscence by Conrad, Joseph

A French dictionary defines inexpressibles as follows - Mot anglais dit par euphémisme pour culotte, pantalon, et employé quelquefois en ce sens en français par plaisanterie (An English word spoken as a euphemism for pants or trousers and sometimes used jokingly in this sense in French.)