look up any word, like wyd:
Demonheart is the name of a song by power/fantasy metal artist Luca Turilli. To say that someone has a demonheart is to say that a person may seem amazing, but they will completely screw you over when you least expect it, typically in a horrible way.
Don't go out with that chick, she has a demonheart.
by fuzzfuzz April 03, 2010