Someone who hangs around you like a bad smell, following you everywhere you go without encouragement, copying whatever it is you do in the hope that your good fortune might rub off on them.
For instance, you go fishing and you find this really good remote spot, out on a rock all by yourself, miles from anyone else. Then a goat's milk comes along and wants to share the exact same space in the hope that they'll get to catch a fish too.
(This is a saying that originated in the 1940's I believe. I learnt this saying from my parents when I was a kid, whenever I told them about the loser kid at school who was hanging around
"Uh-oh, here's comes goat's milk, quick, let's hide!"