also scallawag, "disreputable fellow," by 1839, American English colloquial, of uncertain origin; perhaps an alteration (by influence of wag "habitual joker") of Scottish scallag "farm servant, rustic," itself an alteration of Scalloway, one of the Shetland Islands, the reference being to little Shetland ponies (an early recorded sense of scalawag was "undersized, ill-fed, or worthless animal," 1854).
Judge Lynch passed through town on Saturday night last. He remained here long enough to give a worthless scalawag a genteel suit, from "head to heels" of tar and feathers. [Maumee City Express, Saturday Aug. 3, 1839]
In U.S. history, used from 1862 as a derogatory term for anti-Confederate native white Southerners.
The word was used in the southern United States, during the period of reconstruction (1865 to 1870 and later), in an almost specific sense, being opprobriously applied by the opponents of the Republican party to native Southerners who acted with that party, as distinguished from carpet-bagger, a Republican of Northern origin. [Century Dictionary]