A warlock is a witch/wizard/druid/etc.. who has betrayed their coven/group. It means 'traitor' or 'liar'. Back then it was a witch who betrayed others to the witch hunters. Most non-magick practicing people call male witches this and most Wiccans call people who have been initiated and who betray the art this.
'The Grimoire of the Apprentice Wizard' by Oberon Zell-Ravenheart
(Sorry I didn't copy the definition word for word, I just put the most important parts in)