Proto-Indo-European root meaning "to let go, slacken."
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."
"process of shaking; downfall, overthrow," 1610s, noun of action from Latin labefactus, past participle of labefacere "to cause to totter, shake," literally and figuratively; also "to overthrow," from labare "to totter, stand unsteadily, be ready to fall, begin to sink, give way" (which is perhaps related to labi "to glide, slip, slide, sink, fall; see lapse (n.)) + facere "to make, do" (from PIE root *dhe- "to set, put").
Alternative labefactation (from Latin labefactitionem "a shaking, loosening," noun of action from past-participle stem of labefacere) is attested from 1775. As a verb, labefact is from 1540s, labefy 1620s, labefactate from 1650s.