When you hook up with someone just to have a little fun, nothing serious. Sometimes used in a bad way from the person who was hoping it was more then just a hook up. Used in the Who movie quadrophenia so my guess its of British origin
Nic: Hey so whats up with you and Colette?
James: Ah nothin just having a little giggle
Colette: Wtf? I liked you!! What am i to you? some giggle?!?!?!