undertrousers

From under- +‎ trousers.

undertrousers pl (plural only)

1. Synonym of underpants
   • 1896 May 24, “Some of the New Bathing Suits”, in The Chattanooga Sunday Times, Chattanooga, Tenn., page 18:

     This is composed of a short skirt and high gathered bodice, all in one, with full undertrousers in a separate piece.

   • 1897 July 9, “Feet Were Tied. German Lost His Life While in Swimming. Dropped Into Water—Underdrawers Wrapped Around His Akles—Weeds Closed About Him.”, in Buffalo Courier-Record, volume LXII, number 187, Buffalo, N.Y., page 5:

     Roman wore a pair of loose undertrousers and slowly let himself into the water from the stern of the boat.

   • 1899 August 19, “Fashions for Children. There is Always Something New for Boys and Girls. Simplicity is the Keynote for Everything—Some Attractive and Serviceable School Frocks.”, in The Evening Star, number 14,500, Washington, D.C., page 8:

     Only undertrousers or drawers should be worn instead of petticoats. Indeed, petticoats are not to be included in a boy’s outfit after he is two years old.

   • 1988, Exploring World Partnerships in Technology: 9th International Symposium on Contamination Control: Proceedings, page 487:

     Two sets of clean room garment systems were used and tested: One clean room coverall having high filtration performance, and one set of cotton underwear and undertrousers, and one set of polyester underwear and undertrousers.

   • 1992, Mary Flanagan, Rose Reason, Harcourt Brace Jovanovich, page 304:

     He lifted his skirts and there I saw his undertrousers. They were white and came to his knees. And his red socks also, held up with garters. It was so funny. He took from the shelf another pair of undertrousers and put them on.

   • 2013, Angela Knight, Jennifer Ashley, Jean Johnson, Hanna Martine, Unbound, Berkley Sensation:

     “It’ll go away—keep your knickers on,” he added, shifting off the bed and casting around for his own undertrousers. […] “I never once doubted that, Mi—Vielle,” he promised her, fastening his undertrousers in place.