sickly adj.

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.

suffix forming adjectives from nouns and meaning "having qualities of, of the form or nature of" (manly, lordly), "appropriate to, fitting, suited to" (bodily, earthly, daily); irregularly descended from Old English -lic, from Proto-Germanic *-liko- (Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (Old English lich "corpse, body;" see lich, which is a cognate; see also like (adj.), with which it is identical).