Margo--

The verse about Sally's arse cheeks is both part of "The Hogeye Man" (see the lyrics posted in Digitrad) and of the chantey Poor Old Man (also sometimes called Poor Old Horse), which has a refrain structure like this:

Sally's in the kitchen and she's making the duff ref: and we say so, and we hope so And the cheeks of her arse are going chuff, chuff, chuff ref: Oh poor old man

Isn't folk music wonderful?