DASTARDLY

dastardly, dastard(a)

(adjective) despicably cowardly; “the unprovoked and dastardly attack by Japan on...December 7th”- F.D. Roosevelt

Source: WordNet® 3.1

Etymology

Adjective

dastardly (comparative dastardlier, superlative dastardliest)

in the manner of a dastard; marked by cowardice; pusillanimous

treacherous; given to backstabbing

Adverb

dastardly

In a cowardly or treacherous fashion.

Source: Wiktionary

Das"tard*ly, a.

Definition: Meanly timid; cowardly; base; as, a dastardly outrage.

Source: Webster’s Unabridged Dictionary 1913 Edition