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