The man gets into bed, knocks the bottom boards over the woman's head, gives her his "hairy peg." She likes it. (Refrain: "Lassie, let me in, O") When he comes down, the "auld wife" is standing there; she lifts her clothes and says "Laddie, put it in"
This has a good deal in common with "Let Me In This Ae Nicht," aka "Cold Haily Windy Night," but as the plots are quite different, MacColl & Seeger split them, and so do I. - PJS
I'm glad you added that note, though, or I might have lumped them. (Roud did.) I almost wonder if this isn't "Let Me In This Ae Nicht," with an ending related to "Courting the Widow's Daughter" [Laws H25). - RBW