lightning

Lightning comes from light(e)n plus the suffix -ing, built to name the ongoing action of the verb 'lighten.'

Because it grew straight out of 'lighten,' lightning is a doublet of 'lightening' — the same root split into two spellings that drifted toward two different meanings, one the sudden brightness in the sky, the other the growing lightness of weight or mood.

Its closest relative is 'lightening,' the word for becoming lighter — the two are twin forms of the same origin, not separate coinages.