Donk is an abbreviation for donkey. And a donkey is sometimes seen as a stupid animal. And a donkey can refer to a stupid or silly (ridiculous) person. Such as an ass, which is a synonym for donkey and can also refer to a person that is stupid or silly, as well as a lot of other things.
Thus, you have the donk inserted into ridiculous to come up with redonkulous, a slang word that means, well, as ridiculous as a donkey. Or more than a donkey, even.
Redonkulous can also be spelled redonculous. In this case, it patterns more after the letter c in ridiculous.
Last, there is an associated word, redunkulous (note the u), which has come to mean just a little less silly than redonkulous. At least according to The Urban Dictionary.
Sometimes if all the normal places do not provide a definition, you can try The Urban Dictionary. (Use it at your own risk, since it contains much street slang that includes a lot of profanity.)
The Urban Dictionary will often contain definitions that differ from one another. This means either that the person who has supplied a particular definition is asinine, or that a word has actual meanings that you have never heard of, because they are used as street slang by a specific social-cultural group (thus: urban).
But you can use The Urban Dictionary as a last resort, or just to browse through to see what people think words and phrases mean. Or to see what the latest slang expressions are. The up and down votes are usually a good indicator as to the reliability of the given definition.
The Urban Dictionary definition of redonkulous is spot on. You can also look up spot on in The Urban Dictionary. But you can't believe everything you read, so without further research I am not sure if the origin if redonkulous that the first definition gives is true.
PS--Thanks for the screen shots from the movie.