Oxford English Dictionary, (online) 2nd Edition (1989): "ON. varðlokkur wk. fem. pl. ... incantation, suggested already in Johnson, is too rare (? occurring once), with regard to the late appearance of the -k forms, to be considered."
oxforddictionaries.com
“Definition of warlock”. English Oxford Living Dictionaries. Oxford University Press. 29 December 2018閲覧。