Something felonious is against the law, or related to crime. A felonious spree might involve stealing cars and robbing banks.
Criminal activity is felonious, and if you know someone who shoplifts or steals credit card numbers, you could call him your felonious acquaintance. If a jewelry store manager forgets to lock the shop door, it's a felonious opportunity, unless a passing thief restrains her felonious impulses. Felonious and the related felony come from the Old French felonie, "wickedness, treachery, or crime," from the Gallo-Roman fellonem, "evil-doer."