Etymology
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sickness (n.)

Old English seocnes "sickness, disease; a disease;" see sick (adj.) and -ness. Formerly synonymous with illness; in late 19c. it began to be restricted to nausea, leaving illness as "a rather more elegant and less definite term" [Century Dictionary].

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Definitions of sickness

sickness (n.)
impairment of normal physiological function affecting part or all of an organism;
Synonyms: illness / unwellness / malady
sickness (n.)
defectiveness or unsoundness;
drugs have become a sickness they cannot cure
a great sickness of his judgment
sickness (n.)
the state that precedes vomiting;
Synonyms: nausea
From wordnet.princeton.edu