To put a layer of toilet paper on the seat before dropping a duece.
Alright, so I'm at this baseball game and I have to take a hardcore shit, and I go in the bathroom and it's full to the brim. I get in line for the shitter, and I wait for what seems like hours. The guy in front of me is finally up to plop, and he goes in there and I hear him build a nest. I'm like "what the fuck are you doing, you loony piece of shit?" and he's all like "yeah, whatever."
The act of lining a public toilet seat with toilet paper, usually one ply. the thickness of the nest varies according to each persons preference and/or depending how much piss and shit are on the seat.
i always build a nest before taking a shit in a public bathroom.