Prov. Someone who is used to doing things a certain way cannot change. (Usually not polite to say about the person you are talking to; you can say it about yourself or about a third person.) I've been away from school for fifteen years; I can't go back to college now. You can't teach an old dog new tricks. Kevin's doctor told him not to eat starchy food anymore, but Kevin still has potatoes with every meal. I guess you can't teach an old dog new tricks.