RDN was voiced by Bill Maher on Real Time (5/21/10). Synonymous with PDN (see PDN - Proctologically Derived Number; previously posted to Urban Dictionary); numbers, statistics, budgets, etc. derived from and consisting solely of voluminous fecal accumulation, frequently embellished with politics.
Someone who is stupid, and who doesn't think before they act/speak. Spoonerism of "mentally retarded," and has come to mean someone who isn't retarded, but acts like it.