Origins: the adjective root, ridonkulous, evolved from the ithyphallic wordplay of replacing the "dic{k}" sound in ri·dic·u·lous with "dong" (the "g" becoming silent). The spelling of the root using a "k" instead of a "c" results from the common misconception that the word donkey is somehow related.
An adjective to describe something that is worthy enough to be called exceedingly preposterous that is deserves an even more absurd name than ridiculous.
Person 1: Oh my god! Did you hear that Ashley Olson is dating Lance Armstrong?
Person 2: That is absolutely ridonkulous he's almosttwice her age!