1560s, "showing despicable cowardice," originally "dull," from Middle English dastard + -ly (1). Related: Dastardliness (1550s).
Entries linking to dastardly
dastard n.
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.
-ly 1
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).