The action of taking a crap, shit or piss in a place where there are no proper facilities, such as a toilet or tissue paper. It is usually urgent, when there is no time to seek out better accomodations. Sometimes it is uncontrollable, which makes for a better story to your friends at cocktail parties.
1) I made a mistake of eating a box of prunes before I went Mountain Biking. I had to "go caveman" in the woods with leaves and bark.
2) Man did we drink too much last night after work! I had to "go caveman" in the subway.
3) There was no paper in the outhouse; I had to wipe my ass with my underpants and left them in the garbage can.