In the 1800s, it came to be that the educated person did not use the words trousers, breeches, or (in the US) pants in polite company. Now, as you might imagine, there are a number of situations in which you might want to refer to this item of clothing (in a very polite sort of way, of course). Thus, a number of euphemisms sprang up. One might refer to inexpressibles, inexplicables, ineffables, or unmentionables. It's hardly a surprise that 'unmentionables' won this contest, and is the only one used today.
"Corinthians and exquisites from Bond Street, sporting an eye-glass, ... waiting-men in laced coats and plush unmentionables of yellow, green, blue, red, and all the primary colours.
-- Rev. N. S. Wheaton: Journal (1830).
But eventually people realized that trousers weren't sinful, and it was actually quite proper to speak of them, even in front of the women and children. But it was a pity to let a good euphemism go to waste, and there was another enemy of the prudent conversationalist, the dreaded undergarment. Sometime around 1900, unmentionables slid under the trousers and skirts of the English speaking world, and started referring judiciously to underwear of all sorts. (Webster1913 was no doubt aware of this trend, but was too dignified to chronicle it).
These days we occasionally use unmentionables to refer to undergarments, particularly feminine undergarments, but it is again falling out of use, as Victoria's Secret and other purveyors of the indelicate desensitize us to yet another layer of clothing. You will occasionally see unmentionables used as a euphemism for genitals, although this is still somewhat rare. Perhaps in a decade or two it will catch on. In the meantime, it is usually just a quaint way of saying underpants.
Horsefeathers (& other curious words) by Charles Funk