File System Consistency Check, or File System check is a system utility on Unix systems for checking file consistency.
Microsoft equivalents would be scandisk and chkdsk
It has been used as a profanity, as a replacement of fuck to get past autocensors/filters and also as a literal insult.
Sometimes used as general slang for to check/repair something.
A moron's way of censoring fuck, in order to get around online censors. The funny thing is, most censors will catch poor fsck, but won't catch f*ck. It's seen quite often on popular sites that don't even have censors.