"Sweet Polly Oliver" is an English broadside ballad (Roud #367), traceable from 1840 or earlier.
It is one of the best known of a number of folk songs describing women disguising themselves as men to join the army to be with their lovers.
Thomas Root wrote a symphonic band arrangement and Benjamin Britten wrote an arrangement for voice and piano.
The main theme of Terry Pratchett’s book, Monstrous Regiment, in which a young woman named Polly, who has heard the song sung in her father's inn, joins the army, as a man, to find her brother, taking the name Oliver.
Several versions of "Polly Oliver" survive as undated broadside ballad sheets in the Bodleian Library, University of Oxford.