mid-15c., a term of contempt for one who is lazy or dull; an English formation on a French model, probably from *dast, "dazed," past participle of dasen "to daze" (see daze (v.)) or the equivalent past participle in Old Norse + deprecatory suffix -ard. Meaning "one who shirks from danger, base coward" is late 15c.
suffix forming adjectives from nouns and meaning "having qualities of, of the form or nature of" (manly, lordly), "appropriate to, fitting, suited to" (bodily, earthly, daily); irregularly descended from Old English -lic, from Proto-Germanic *-liko- (Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (Old English lich "corpse, body;" see lich, which is a cognate; see also like (adj.), with which it is identical).
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Definitions of dastardly from WordNet
despicably cowardly; "the unprovoked and dastardly attack by Japan on...December 7th"- F.D. Roosevelt;