(n) Used to describe someone whose positive attributes, such as enthusiasm, friendliness or good looks, are completely undone by their mind boggling stupidity. The type of person who would do anything to please you if only they could remember what it was. Or who, in their excitement, is likely to urinate right where they stand if you say something nice to them or feed them a snack.
"Mate, that bloke's got a touch of the Border Collie about him."
"She was up for it, but I had to walk away when she went all Border Collie on me."