dastardly UK /ˈdɑːstədli/US /ˈdæstɚdli/adjective 2adverb 1forms 2 adjective2 •In the manner of a dastard; marked by cowardice; pusillanimous.I ask that the Congress declare, that since the unprovoked and dastardly attack by Japan on Sunday, December 7th, 1941, a state of war has existed between the United States and the Japanese Empire. — 1941 •Treacherous; given to backstabbing. adverb1 •In a cowardly or treacherous fashion. formsdastardlier · dastardliest