RAR! is a board meme coined by Mayabird in response to Zor's "Hipotheticle Senareos"[sic].
What is a RAR!
A RAR! thread refers to a thread with a "highly complicated" hypothetical scenario. Originally it was a purely derogatory remark (and is occasionally still used as such). Now it is a somewhat well known aspect of SD.net Culture and has taken on the role of simply describing a hypothetical scenario that may or may not be stranger than your usual ones. Mere versus threads are not considered RARs, however RARs can include versus elements inside them.
RAR is not an acronym, so stop asking what it stands for. It is also a type of downloadable file.