O.E. cwealm (W.Saxon) "death, disaster, plague," utcualm (Anglian) "utter destruction," related to cwellan "to kill," cwelan "to die" (see quell
). Sense softened to "feeling of faintness" 1530; meaning "uneasiness, doubt" is from 1553; that of "scruple of conscience" is 1649.
A direct connection between the O.E. and modern senses is wanting, but it is nonetheless plausible, via the notion of "fit of sickness." The other suggested etymology, less satisfying, is from Du. kwalm "steam, vapor, mist," which also may be ult. from the same Gmc. root as quell.