From over + -ling on analogy with underling, or as a revived or dialectal term from Middle English overling (“a superior”), equivalent to over + -ling.
"overling." YourDictionary, n.d. Web. 17 August 2018. <http://www.yourdictionary.com/overling>.
overling. (n.d.). Retrieved August 17th, 2018, from http://www.yourdictionary.com/overling