sick + -er
From Middle English siker (“secure, safe, stable, certain; gewiss, securely, safely, certainly”), from Old English sicer, sicor (“secure from, free from guilt and the punishment, safe, free from danger or harm, sure, certain, free from doubt, trustworthy”), from Proto-Germanic *sikuraz (“free, secure”), from Latin sēcūrus (“secure”, literally “without care”). See secure. Cognate with Scots siker, seker (“safe, secure”), North Frisian sijcker (“sure, secure”), Dutch zeker (“sure, certain, safe, secure, confirmed”), German sicher (“sure, secure, confirmed”), Swedish säker (“secure, safe, sure”), Norwegian sikker (“secure”).
(third-person singular simple present sickers, present participle sickering, simple past and past participle sickered)