If you acquire something in a dishonest way — by stealing it, for example — it's ill-gotten. When a bully pressures classmates to hand over their lunch money, the bully might go home that day with a pocket full of ill-gotten quarters.
You'll often find this adjective in the phrase "ill-gotten gains," a general description of anything that's obtained unethically. The CEO of a company who embezzles money from it might be caught with a bank account full of ill-gotten gains, and pawn shops are places where criminals sometimes sell the ill-gotten gains they've stolen. If your dog steals your dinner off the kitchen table, you can describe it as his ill-gotten steak.