Yours, Etc.: Origins and Uses of 8 Sign-Offs
late 14c., "track cut through a wood," later "long, narrow ditch" (late 15c.), from Old French trenche "a slice, ditch" (late 13c.), from trenchier "to cut," possibly from Vulgar Latin *trincare, from Latin truncare "to cut or lop off" (see truncate). Trenches for military protection are first so called c.1500. Trench warfare first attested 1918. Trench-coat first recorded 1916, a type of coat worn by British officers in the trenches.
A long, steep-sided valley on the ocean floor. Trenches form when one tectonic plate slides beneath another plate at a subduction zone. The Marianas Trench, located in the western Pacific east of the Philippines, is the deepest known trench (10,924 m or 35,831 ft) and the deepest area in the ocean.