Boulder Build-Up is when someone (usually over weight men and women) have dirt and sweat build up under or between cracks in there body from limited exersise and movement. Normally occurs under your arms, between your legs, under your flab (fat) or in more sever cases just in open areas. This dirt build up sometimes falls out in large chunks often named Boulder Build-Up.
Bob: "Oh gee i really need to go to the toilet"
Bob: (mind) ill just lif up my fat and do my buisness.
Bob: "OMG, what is that stuff"
Bob: "I think i have Boulder Build-Up!!!"