Sickness

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Old English sēocnesse (see sick1, -ness).

wiktionary

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From Old English sēocnes. Synchronically analyzable as sick +‎ -ness.

etymonline

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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].