sickness n.

Middle English sik, from Old English seoc "ill, unwell, diseased, feeble, weak; corrupt; sad, troubled, deeply affected by strong feeling," 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 it was displaced by krank "weak, slim," probably via the notion of "twisted, bent" (see crank (n.)).

The restricted meaning of English sick, "having an inclination to vomit, affected with nausea," is from 1610s. By c. 1200 as "distressed emotionally by grief, anger, etc.; physically ill through emotional distress. The sense of "tired or weary (of something), disgusted from satiety" is from 1590s; the figurative phrase sick and tired of is attested from 1783. To worry (oneself) sick is by 1952.

The modern colloquial meaning "mentally twisted" is by 1955, a revival of the word's use in this sense from 1550s (the sense of "spiritually or morally corrupt" was in Old English, which also had seocmod "infirm of mind"). Sick joke is attested by 1958.

word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).

"disease, sickness, ailment, malady," 1680s, from ill (adj.) + -ness. Earlier it meant "bad moral quality" (c. 1500).