A great muckle skate is a great muckle Pandora's box because a skate is a fish of temperate seas. Yet it is associated with an old fish hook in this song. However we are not talking river estuary here, because the skate, like the lady astride of the gate, floated past village and cottage and town. So how did a salt water fish get in the upper reaches of a river? On the other hand a "skate" is a contemptuous term for a stupid or objectionable person, as we gather the floating lady was from her crotchety remark to the fisherman (or sailor in my version). But could not the great muckle skate be a great muckle slidy thing you strap on your foot? No, because there's food and there's raiment gaun doun tae the sea. The raiment is the top hat. The food must be the skate, perhaps with an intentional self-depracatory pun by the song's stupid author.