What does Boxing Day have to do with boxing?
1650s, "relaxing, soothing," from Middle French lenient, from Latin lenientem (nominative leniens), present participle of lenire "to soften, alleviate, mitigate, allay, calm," from lenis "mild, gentle, calm," probably from PIE root *le- "to leave, yield, let go, slacken" (cf. Lithuanian lenas "quiet, tranquil, tame, slow," Old Church Slavonic lena "lazy," Latin lassus "faint, weary," Old English læt "sluggish, slow," lætan "to leave behind"). Sense of "mild, merciful" (of persons) first recorded 1787. In earlier use was lenitive, attested from early 15c. of medicines, 1610s of persons.