1. <operating system> file system check. The Unix program that checks a file system for internal consistency and bad blocks etc. and can repair some faults.
fsck is often used after a crash when the file system has been left in an inconsistent state, e.g. due to incomplete flushing of buffers.
2. <jargon> Used on Usenet newsgroup alt.sysadmin.recovery as substitute for "fuck" and became more main-stream after the Communications Decency Act.