It pretty much means the exact same thing as laughable and makes for a good alternative. Kansas University supporters' attempts to justify the actions of Silvio De Sousa were risible. As a non American I find the idea of every individual needing to own a gun for their personal protection quite risib...

This is such a lovely word, almost onomatopoeic. I am too lazy to look it up but I think it was used to describe Passeltongue in the Harry Potter books. I have seen in almost exclusively used as an adjective for "whisper".