no-brainer / nəʊˈbreɪnər /

Homer Simpson,
a no-brainer (almost literally)

It is very funny that English seems to have 'no-brainers' even though there are no 'brainers' of any kind. How can we guess the meaning of the negation of something that doesn't exist?

Well, actually it is not that difficult: a no-brainer is not a person, but a question or, especially, a decision so clear that even someone with no brains can get it right.

Example 1: http://www.bythom.com/2011%20Nikon%20News.htm

Touchfire is a clear overlay that makes the on-screen iPad keyboard feel like a real keyboard. I don't know how to say it any simpler. At one ounce and iPad Smart Cover friendly, it's sort of a no-brainer if you do any significant typing on an iPad, as you don't need to add an external keyboard to your kit

Example 2: Arts Beat - It’s a No-Brainer: AMC Orders Third Season of ‘The Walking Dead’
In this case the expression is in the title of this blog post. As it goes on, it lets you understand why this is a no-brainer:

After a recent episode of “The Walking Dead” that vividly revealed the contents of a zombie’s stomach, AMC has decided it still has a healthy appetite for that popular series about the survivors of an undead apocalypse. [...]The second-season premiere of “The Walking Dead,” broadcast on Oct. 16, set a ratings record for AMC, drawing 7.3 million viewers in total and 4.8 million were ages 18 to 49, making it the highest-rated drama episode ever on cable in that age category.

Example 3: Finally, The Phrase Finder discusses the origin of no-brainer in the 50's, and even contains a cartoon by Carl Grubert showing an early example in the Long Beach Independent (1959)

Unfortunatelly, despite knowing the meaning of no-brainer, I don't find it very funny. Maybe I am missing something else.