Describes when you're on some form of public transportation (bus, train, etc) and you're uncomfortable from the people on your right and left. Used particularly to describe frustration when the people on the right or left can make some more room for you but don't.
A: You look beat!
B: Yeah, I was stuck in a seat sandwich for the full 2 hour train ride here. The idiot on my left had room but he wouldn't budge. I hate public transit!