From the AskOxford site "The word lukewarm has a long and respectable history, going right back to the 14th century. The adjective luke sems to be an alternative form of lew, Old English hleow, which meant 'tepid'"
Its in the Bible Revelation Chapter 3. New testament, a well known saying would rather have someone hot or cold but not lukewarm meaning would rather someone nice to you or not but not in between not knowing how they feel.about you