"frighten, terrify, alarm," mid-15c.; see a- (1) + fright (v.). It probably was back-formed from older affright (adj.) "struck with sudden fear" (which is metathesized from Old English afyrht, past participle of afyrhtan "to frighten, terrify"). The doubled -f- is 16c., probably an erroneous Latin correction of a non-Latin word (compare afford). Related: Affrighted; affrighting; affrightment.
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.