We used to have a little Lancashire terrier dog which on occasions used to tear round the house like a wild thing at great speed for no apparent reason. Backwards and forwards it rushed expending excess energy just for the sake of it. My Mother used to say, "it's having a light do" meaning it was behaving rather bizarrely. We all knew what she meant but I've often wondered what was the origin of this phrase. Has anyone heard this and can anyone please explain?