mid-15c., of persons, "mild in temper or disposition" (attested from early 13c. as a surname), from Old French clement, from Latin clementem (nominative clemens) "mild, placid, gentle" (see clemency). Of weather, 1620s. Taken as a name by several early popes and popular in England as a masculine given name from mid-12c., also in fem. form Clemence.
1650s, "relaxing, soothing" (a sense now archaic), from French lenient, from Latin lenientem (nominative leniens), present participle of lenire "to soften, alleviate, allay; calm, soothe, pacify," from lenis "mild, gentle, calm," which probably is from a suffixed form of PIE root *lē- "to let go, slacken."
The usual modern sense of "mild, merciful" (of persons or actions) is first recorded 1787. In earlier use was lenitive, attested from early 15c. of medicines, 1610s of persons. Related: Leniently.
"softness, smoothness, mildness," early 15c., from Old French lénité or directly from Latin lenitatem (nominative lenitas) "softness, smoothness, gentleness, mildness," from lenis "soft, mild" (from PIE root *lē- "to let go, slacken").