early 15c., from Middle French lassitude (14c.), from Latin lassitudinem (nominative lassitudo) "faintness, weariness," from lassus "faint, tired, weary," from PIE *led- "slow, weary" (source also of Old English læt "sluggish, slow;" see late, adj.), from root *le- "to let go, slacken" (see lenient).