The story
Analyzable as lenis + -ition, or as if from Latin lēnīt(us) + -ion, or Latin lēnītiō (“softening”) from lēniō (“soften”) + -tiō (action noun suffix) (attested since at least the 1500s, the same timeframe lenition is first attested in English with the sense "assuaging"). Modelled on German Lenierung.
Etymology from Wiktionary, CC BY-SA 4.0.