late 14c., "track cut through a wood," later "long, narrow ditch" (late 15c.), from Old French trenche "a slice, cut, gash, slash; defensive ditch" (13c., Modern French tranche), from trenchier "to cut, carve, slice," possibly from Vulgar Latin *trincare, from Latin truncare "to maim, mutilate, cut off," from truncus "maimed, mutilated," also "trunk of a tree, trunk of the body," of uncertain origin, probably originally "mutilated, cut off," and perhaps from PIE root *tere- (2)"cross over, pass through, overcome."
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 during World War I.