Bastard is a curse word which means ‘someone whose parents were not married to each other when he or she was born; in other words, an illegitimate child’. An extension of this meaning is the figurative usage of bastard , it is used to refer to something that is of irregular, inferior, or of dubious origin; impure, irregular, inferior: “It was a bastard style of painting that completely turned off the entire gathering. ”
Dastard, on the hand, is an adjective which refers to a sneaking & malicious type of coward: “Money can buy almost everything but it buy you good-heartedness, a dastard would always remain one.”