In an unfaithful manner; without faithfulness; perfidiously; negligently: as, work unfaithfully done.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
ad. in an unfaithful undependable unreliable manner
Word Usage
"But Catherine suspects her husband of infidelity, and so makes the unusual move of hiring a prostitute, the eponymous Chloe Amanda Seyfried to attempt to seduce David and see if he reacts unfaithfully."