If someone calls you mephistophelian, they're commenting on your devilish nature or your gleeful wickedness. For example, the supervillain known as The Joker has a particularly mephistophelian laugh.
The adjective mephistophelian is a great way to describe an evil genius or a clever, wicked person. The word comes from a demonic character in German folklore, Mephistopheles, whose name stems from mephitz, "destroyer" in Hebrew, and tophel, "liar." Your little brother might look mephistophelian in his Halloween devil costume, and you could even describe a perfectly nice man with a pointy beard as mephistophelian.