When one uses one's fingers to bash a pie, tart , jelly , vagina, or any other squishy slimey object.
Girl- ' oi jim ! What the hell are you doing to my flan!'
Jim- ' oh . . i'm just fingerbashing it?'
Girl- ' why?'
Jim- 'It feels good . . . do you want to fingerbash you? '
Girl- ' Sure'