Where the man puts his penis in a girls armpit, whereupon she proceeds to lift her arm up and down, like she is playing the bagpipes. This act is known as Bag-piping.
"We had a nudy ceildhi last night and had a good ole time Bag-piping"
Where you hold your forskin closed, ejaculate into your forskin to create a piping bag of sorts, then proceed to decorate your partners breasts/face/wedding cake/gingerbread man etc with your semen.
Barry: that is some decorative calligraphy on that cake