The act of searching out somebody who has just achieved something good and then instantly ruining it for them by doing something annoying to take the shine off of it. Usually a whole day activity as finding somebody who has just achieved success isn't easy.
If someone gets a good grade in an exam, throw a pint of milk in their face.
If a kid learns to ride a bike for the first time, run up and push it over.
You get the idea.
Hey John, fancy a spot of bastarding today?