shoo v.

admonition to a pest, by 1866 (in political journalism), from shoo (v.) + fly (n.). Popularized by a Dan Bryant minstrel song ("Shoo fly — don't bother me") that appeared in print by 1868 and was enormously popular from the next year, which launched it as a catch-phrase that, according to H.L. Mencken, "afflicted the American people for at least two years" and made an appearance in the Congressional Record.

[A]n epidemic called the "Shoo fly," that has raged in New York and other eastern cities for some time, has at length broken out there [Cincinnati] with great intensity. Young and old have got it. Babies lisp it, and old men are heard to pipe it forth. The child, when threatened with chastisement by its mother, modifies the parental wrath, and sometimes escapes the impending rod by waving its infantile hand and singing out:"Shoo-fly, don't bodder me!"

[Knoxville, Tenn., Daily Press and Herald, Jan. 28, 1870]

 The phrase also was used in various technical senses. The Pennsylvania Dutch delicacy shoo-fly pie is attested by 1908 in a recipe in the Philadelphia Inquirer.

Its name is generally taken to be an allusion to the fact that it is so attractive to flies that they have to be constantly shooed away from it, but the fact that it originated as a Pennsylvania-Dutch specialty suggests the possibility that shoofly is an alteration of an unidentified German word. [Ayto, "Diner's Dictionary"]

Beam's "Pennsylvania German Dictionary" [Lancaster, 1985] gives der Melassichriwwelboi as the translation of "shoofly pie."

"easy winner" (especially in politics), 1939, from earlier sense of "horse that wins a race by pre-arrangement" (1937), from the verbal phrase shoo in "allow to win easily" (1908); see shoo (v.) + in (adv.).