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."

Proto-Indo-European root meaning "to let go, slacken." 

It forms all or part of: alas; allegiance; lassitude; last (adj.) "following all others;" late; latter; lenient; lenitive; lenity; let (v.) "allow;" let (n.) "stoppage, obstruction;" liege.

It is the hypothetical source of/evidence for its existence is provided by: Greek ledein "to be weary;" Latin lenis "mild, gentle, calm," lassus "faint, weary;" Lithuanian lėnas "quiet, tranquil, tame, slow," leisti "to let, to let loose;" Old Church Slavonic lena "lazy," Old English læt "sluggish, slow," lætan "to leave behind."