A combination of the words belly and shelf. The "shelf" that is made by a person with belly or a pregnant woman when they sit down. The top of their belly can hold a cup in leiu of using the coffee table.
I can put this cup on my belf while I text since there is not a table.
The other day I belfed. I totally threw up but I was in line at the grocery store and just couldn't let it all come flying out onto the floor, so I swallowed it.