English word sickness comes from Old English seocness

Detailed word origin of sickness

Dictionary entry	Language	Definition
seocness	Old English (ang)
sickness	English (eng)	(linguistics) The analogical misuse of a rarer or marked grammatical case in the place of a more common or unmarked case.. Nausea; qualmishness; as, sickness of stomach.. The quality or state of being sick or diseased; illness.