I believe pig is a word, not an acronym, although maybe it's a bacronym now.

https://en.wikipedia.org/wiki/Pigging

> Some early cleaning "pigs" were made from straw bales wrapped in barbed wire[4] while others used leather.[5] Both made a squealing noise while traveling through the pipe, sounding to some like a pig squealing,[6] which gave pigs their name.[7][8]