This term refers to being on your best behavior when doing bad ass masculine shit like climbing rugged peaks so people put up with you and don't push you off a cliff.
There are many unspoken rules of good Expedition Behavior such as:
Don't fart in the tent.
Try to retain some hygiene- other people have to look at and smell you.
Don't be a lazy ass- carry your own weight.
Don't be cocky.
Joe was not on his best Expedition Behavior. He chewed his ramen with his fucking mouth open, smelled like a rotten llama, and bitched about his hangnail while we were summiting the north face of K2. Therefore Joe is no longer with us- he never made it down the mountain and there were no witnesses.
Somewhere between a skittle thug and what you would find under flat brim. But most applicable when applied to downhill mountain bikers. They have an affinity for chilling at the trailhead bumping gansta rap and puffin' tuff and are physically incapable of getting their $5,000 bicycle up the hill so must coordinate with a cohort to shuttle themselves up and down the mountain with their two $30,000 pick up trucks. As you can see, this becomes a financially burdensome endeavor, especially when you take into account the 5 Monsters they consume per day.