c. 1200, "to become ill," from sick (adj.) + -en (1). Transitive sense of "to make sick" is recorded from 1610s. Related: Sickened; sickening. The earlier verb was simply sick (Old English seocan) "to be ill, fall ill."
"unwell," Old English seoc "ill, diseased, feeble, weak; corrupt; sad, troubled, deeply affected," from Proto-Germanic *seuka-, which is of uncertain origin. It is the general Germanic word (compare Old Norse sjukr, Danish syg, Old Saxon siok, Old Frisian siak, Middle Dutch siec, Dutch ziek, Old High German sioh, Gothic siuks "sick, ill"), but in German and Dutch displaced by krank "weak, slim," probably originally with a sense of "twisted, bent" (see crank (n.)).
Restricted meaning "having an inclination to vomit, affected with nausea" is from 1610s; sense of "tired or weary (of something), disgusted from satiety" is from 1590s; phrase sick and tired of is attested from 1783. Meaning "mentally twisted" in modern colloquial use is from 1955, a revival of the word in this sense from 1550s (sense of "spiritually or morally corrupt" was in Old English, which also had seocmod "infirm of mind"); sick joke is from 1958.
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.