Dastardly means wicked and cowardly, the kind of sneaky, underhanded behavior that makes you think someone is both mean and afraid to face the consequences. A dastardly villain in a story doesn't just do bad things: they do them in sneaky, cowardly ways, like tricking others or attacking when no one's looking.

The word carries a feeling of contempt because it combines two negative qualities. Someone might be brave but cruel, or cowardly but harmless, but a dastardly person is both cruel and sneaky. In old adventure stories, the villain who ties someone to the railroad tracks and runs away cackling has committed a dastardly deed.

Today, people often use dastardly playfully or dramatically, like calling your friend's move in checkers a “dastardly trick” when they capture three of your pieces at once. But the word still suggests something both wrong and cowardly: the kind of action where someone does something mean and then hides from responsibility. A dastardly plan succeeds through deception rather than courage or honest effort.