One of the 12 tribes of "Dude," including any and all of the appropriate relational branches of "The Dude," "El Duderino," etc, whom you identify as one of your fellow tribe, for whatever reason.
Like many other tribes, this kinship can create some strong bonds. Membership requirements, and requirements in general, are...abiding, and otherwise quite a bit relaxed by comparison, not dissimilar to a Ralph's Value Club Card membership.
*Note: A strong mutual disdain for unauthorized urination on living space centerpieces (particularly the soiling of prized sectional throw rugs), may be shared at some point in the formation of this bond, but this is not required.
Ancient lore (Duderonomy 1:1) suggests that after parchments had become oversaturated with "bro" and its variants, (including considerable usage of "Broseph") relevant edicts were...you know, "edicted" or whatever is done, and then here we are, more or less...
*For a more cursory understanding of the origin, find yourself, at various points in your life, somehow in the same room with both a flood of ancient scripture and the Mr. Lebowskis of this world and you will be within landing distance of this airport.
*For a considerably thorough tribal history, see the historical works of Dudesephus.
"Nice rip! My Dudeseph just crushed it right into valet parking!!"
"thx Dudeseph!" (original usage)