RFS, an abbreviation for Really Funny Stuff, is a popular phrase that is being used in Internet slang. It is a replacement for the long out dated, and over used "LOL". It is one of many initialisms for expressing bodily reactions, in particular laughter, as text, including initialisms for more emphatic expressions of laughter such as RFFS ("Really Flippin' Funny Stuff")
The list of acronyms "grows by the month" and they are collected along with emoticons and smileys into folk dictionaries that are circulated informally amongst users of Usenet, IRC, and other forms of (textual) computer-mediated communication. These initialisms are controversial, and several authors recommend against their use, either in general or in specific contexts such as business communications.
On May 15, 2011, RFS was formally recognized in an update of the Urban Dictionary.
Guy 1: Hey man, check this funny joke, Two fish swim into a concrete wall. One turns to the other and says "Dam!"
Guy 2: Whoa that's funny, RFS.