lenitive adj.

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."  

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.