RTFM is an acronym for the statement "Read The Fucking Manual." This instruction is usually given in response to a question that can be answered easily by reading relevant documentation, and suggests that the inquirer may be wasting people's time. To avoid the use of the expletive in the statement (which is highly offensive to many people), RTFM also means "Read The Foolish Manual", along with other suggestive words beginning with F. It is also sometimes taken to mean "Read The Friendly Manual" or "Read The Fine Manual" and used less pejoratively.
Derivatives of this found in hacker slang include RTFS ("read the fucking source") and RTFB ("read the fucking binary"). The former expression is also found in a variant inspired by Star Wars: UTSL ("Use The Source, Luke"). These versions are less pejorative than RTFM and used in a humorous sense, as it is assumed that it was the developer's fault for not providing documentation — in particular RTFB is used to indicate that a program is so ancient or poorly documented that the only way to work out what it does is by examining the machine code.