(Sylvie) decides to test her love's faithfulness. Dressed as a (male) robber, she stops him on the road. He gives her his watch and gold, but refuses to hand over his diamond ring. She lets him go, satisfied of his faithfulness, and later reveals herself
According to Patrcik Pringle, _Stand and Deliver: Highwaymen from Robin Hood to Dick Turpin_, chapter 7, "Wicked Ladies," there were a few known instances of female highwaymen during their great era in the seventeenth and early eighteenth centuries.
He mentions specifically Mary Frith ("Molly Cutpurse"), though she was first and foremost a fence rather than a highway(wo)man (to be a highwayman, one had to have a horse, and a pistol generally helped, too). She supposed was the subject of Dekker and Middleton's 1611 play "The Roaring Girls" but her death is dated 1659.
Pringle does not mention a case similar to that in this song.
Jerome S. Epstein, who transcribed the Warner version of Lena Bourne Fish, noted the peculiar tonal peregrination of the tune -- it appears to be in the key of C, but uses all of the following tones (ascending the scale): B C D E F F# G A Bb C. He comments that this sort of modal modulation is very rare in folk song -- but in fact the result, except for that one stray Bb and the ending on C, is pretty close to the Dorian version of "Sovay" I have heard. It sounds to me as if it's a Dorian tune partly and imperfectly moved to Ionian.
Mackenzie's peculiar name for the girl, Zillah, recalls Lamech's wife in Genesis 4:19-23, but I don't know if that is significant. - RBW