"unwell," O.E.
seoc, from P.Gmc.
*seukaz, of uncertain origin. The general Gmc. word (cf. O.N.
sjukr, Dan.
syg, O.S.
siok, O.Fris.
siak, M.Du.
siec, O.H.G.
sioh, Goth.
siuks "sick, ill"), but in Ger. and Du. displaced by
krank "weak, slim," probably originally with a sense of "twisted, bent" (see
crank). Meaning "having an inclination to vomit" is from 1614; sense of "tired or weary (of something)" is from 1597; phrase
sick and tired of is attested from 1783. Meaning "mentally twisted" is from 1551 (though sense of "spiritually or morally corrupt" was in O.E.), revived 1955.
Sick joke is from 1959;
sicko (n.) is from 1977.
Sickening "causing revulsion" is first recorded 1789. The noun meaning "those who are sick" was in O.E.
Sickness is O.E.
seocnesse;
sickly "ailing" is recorded from c.1350.
[sik] 
adjective, -er, -est, noun 
) sick persons collectively (usually prec. by the).