frighten (v.)

Middle English freiht, fright, from Old English (Northumbrian) fryhto, metathesis of Old English fyrhtu "fear, dread, trembling, horrible sight," from Proto-Germanic *furkhtaz "afraid" (source also of Old Saxon forhta, Old Frisian fruchte, Old High German forhta, German Furcht, Gothic faurhtei "fear"). Not etymologically related to the word fear, which superseded it 13c. as the principal word except in cases of sudden terror. For spelling evolution, see fight (v.).

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.