Stands for Simultaneous Mutual Companionable back-clap, and is used to describe the warmly-affable gesture that two super-sociable best friends (usually two cool dudes, but hot chicks occasionally share one, too, or a guy and a sturdy-figured cutie) perform to signify an agreement, encourage each other to begin a boring/difficult/disgusting task, start off on a stroll together, etc. It simply involves exchanging a huge beaming ear-to-ear grin, then putting an arm around each other's shoulders and landing a good-natured thump on the back to indicate their hearty positive feelings and congratulatory appreciation for each other.

When you eat dirt for the first time and leave your family to build a shack in the woods