Is a term used by (usually) pot smokers though it can be used when a group of people are smoking (sharing) anything. The person who "calls" "backs" gets to take second hit. Kind of like calling "shotgun" for passenger seat of a car...
*Todd prepares to spark the blunt..
Rodd: I got backs!
Dodd: Fuck, I got trips
Dick: shit that means I'm last. Technically this is better for me because the smoke from the smoked weed passes through the blunt and deposits THC resin on the bud that it passes through to get the mouth end of the blunt. This means my first hit will yield more THC for me. Yet, it also means that I will have a smaller number of total hits on the blunt. This means I must chief on the blunt like a motherfucker.