As a Noun:
1. An activity done just for the fun of it. English = amusement, fun.
As a Verb or Adjective:
1. To do something just for fun, for amusement. Often something that teases or irritates other people.
1. Used alone to mean that you accept an invitation or suggestion to do something for no particular reason at all. English = 'Oh yes, why not?', 'Yeah, I'm in the mood!', 'That'll be a nice change!' and so on.