A variation of "ridiculous" that plays on the substitution of the "dick" part of the word being replaced by "donk". "Donk" refers to donkeys and thereby jackasses, so the switching is essentially "ass" instead of "dick". Usually used as a more directed attack than "ridiculous". Also used often when relating to a person who is a "jackass" with reference to the movie Happy Gilmore. Variations of "jackass" include "donk", so when the context is such that ridiculous activity and golf are combined thus is "redonkulous" formed.
You're never gonna make that putt, ya jackass!
Oh my god! You made it! That is redonkulous!