To be prepared, mentally and/or physically, for extreme opposition; typically used in reference to an aggressive or potentially violent situation.

The phrase originates with American hunters and woodsmen in regions frequented by the brown bear. Brown bears are the largest land-based predator on earth, and when expecting to deal with them the hunters would bring much more powerful rifles than they would if hunting other game.

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

Carrying more equipment than necessary. Overloaded. It can be used in a variety of ways meaning a person is equipped more than necessary for his/her prey.