"rough, coarse, unkempt," 1590s, from shag, n. + -y(2). Related: Shaggily; shagginess. Earlier was shagged, from Old English sceacgede "hairy;" compare Old Norse skeggjaðr, Danish skægget "bearded." The shaggy-dog story as a type of joke is attested from 1944, perhaps from vaudeville.