Title text: I do this constantly
This is the first "My Hobby" comic in xkcd. In these comics, Randall suggests an obscure activity or pastime he enjoys which he declares as his "hobby". In the premiere "My Hobby" comic, Randall's hobby is mentally re-interpreting what people mean when they say "[adjective]-ass [noun]" by moving the hyphen to after the word "ass" instead of before.
The semi-scatological suffix "-ass" is used as an intensifier in informal US English speech, usually attached to an adjective directly modifying a noun, as in "big-ass car" or "funny-ass comedian". In this comic, Cueball is exploring the increased humor aspect of changing "-ass" from a suffix modifying the adjective, to "ass-", a prefix modifying the noun, yielding a "big ass-car" or a "funny ass-comedian", the former presumably being a large car for carrying buttocks, the latter being a humorous comedian specializing in jokes about lower back. The prefix "ass-" may also have a negative connotation, indicating that something is disliked. An "ass-car" may be a very terrible car, for example.
Another explanation would be that, since this suffix/prefix refers to an element of human anatomy, the car would be in the shape of said anatomical piece.
Outside of North America, most English speakers use "arse" to mean the buttocks, so to them it may sound as if Randall's talking about Donkeys.
On an interesting note; there really is such a thing as an Ass-Car!
- My hobby: whenever anyone calls something an [adjective]-ass [noun], I mentally move the hyphen one word to the right.
- [Cueball is talking to a friend about a car that resembles a Volkswagen Beetle.]
- Cueball: Man, that's a sweet ass-car.
- Original comment from Randall: "Tune in Monday for the conclusion to the story of the boy and his barrel." The comic posted on Monday was 25: Barrel - Part 4, which really wasn't the conclusion of the story of the boy and his barrel.
- This is the twenty-fifth comic originally posted to livejournal. The previous was 23: T-shirts. The next was 25: Barrel - Part 4.