As a clarification of avalyn's node, a shotgun (often shortened to "shottie") originally refers to a seriously hardcore way of taking a hit from a joint.
After blowing off any ashes from the joint's cherry, the first person ("giver") holds the joint and sticks the lit end into hir mouth. That's right, hot end into the mouth — this is why it takes a teensy bit of know-how to do this. The other person ("taker") places hir mouth around the unlit end of the joint, and breathes in as the giver breathes out.
The combination of the giver blowing out and the taker breathing in burns through quite a bit of that joint. (Indeed, many a shotgun has been cut short by flaming ashes burning the giver's lips.) However, this means that the taker gets an impressively large hit from a run-of-the-mill joint.
Shotguns can be fun when indoors (or anywhere else devoid of air motion), because with a little practice the giver can blow streams of smoke short distances.