brighten (v.)

"radiating or reflecting light," Old English bryht, metathesis of beorht "bright; splendid; clear-sounding; beautiful; divine," from Proto-Germanic *berhtaz "bright" (source also of Old Saxon berht, Old Norse bjartr, Old High German beraht, Gothic bairhts "bright"), from PIE root *bhereg- "to shine; bright, white." The meaning "quick-witted, having brilliant mental qualities" is from 1741.

The Germanic word was commonly used to form given names, and figures in the etymology of Robert, Albert, Bertha, Egbert, Gilbert, Herbert, Hubert, Lambert. In modern German it survives in names only (Albrecht, Ruprecht) and has been otherwise lost.

word-forming element making verbs (such as darken, weaken) from adjectives or nouns, from Old English -nian, from Proto-Germanic *-inojan (also source of Old Norse -na), from PIE adjectival suffix *-no-. Most active in Middle English and early modern English, hence most verbs in -en are comparatively recent.