Ey Up! - "snig" seems to have come to mean anything slimy and wiggly! The most common Lancashire definition is "eel", still in use in some quarters, which then makes sense in "Snig Pie". I've been singing this song for years and know what it's about. I would be very interested in the "original MS" mentioned by GuestJ, in June, as the earliest version I've found is in J O Halliwell's 'Dictionary of Archaic & Provincial Words' from the early C19th. It seems to be him that gave it the date of 1548, and all the other Victorian song collectors followed him! I can't prove it but I have a lingering doubt that he may not be right.