Slang word for ridiculous.
Person A: HOLY SHIT I BLEW MY LOAD ALL OVER MY KEYBOARD
Peron B: HHAHA WTF THAT'S RADONKULOUS!
Something over the top and just so awesome it can't even be thought of by the likes of those who were old people in 1982.
...And my headphones were automatically tied in a knot during that fray - RADONKULOUS.
Of course the above fragment only makes sense when put in the context of the musical conversation that preceded it.