stiffen v.

Old English stif "rigid, inflexible," from Proto-Germanic *stifaz "inflexible" (source also of Dutch stijf, Old High German stif, German steif "stiff;" Old Norse stifla "choke"), from PIE *stipos-, from root *steip- "press together, pack, cram" (source also of Sanskrit styayate "coagulates," stima "slow;" Greek stia, stion "small stone," steibo "press together;" Latin stipare "pack down, press," stipes "post, tree trunk;" Lithuanian stipti "to stiffen, grow rigid," stiprus "strong;" Old Church Slavonic stena "wall"). Of battles and competitions, from mid-13c.; of liquor, from 1813. To keep a stiff upper lip is attested from 1815. Related: Stiffly.

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.