sticky —
Description of the `sticky' (S_ISVTX) bit functionality
A special file mode, called the sticky bit (mode
  S_ISVTX), is used to indicate special treatment for
  directories. See chmod(2) or the
  file /usr/include/sys/stat.h
For regular files, the use of mode S_ISVTX is reserved
  and can be set only by the super-user. NetBSD does not
  currently treat regular files that have the sticky bit set specially, but this
  behavior might change in the future.
A directory whose “sticky bit” is set becomes a directory in which
  the deletion of files is restricted. A file in a sticky directory may only be
  removed or renamed by a user if the user has write permission for the
  directory and the user is the owner of the file, the owner of the directory,
  or the super-user. This feature is usefully applied to directories such as
  /tmp which must be publicly writable but should deny
  users the license to arbitrarily delete or rename each others' files.
Any user may create a sticky directory. See
    chmod(1) for details about
    modifying file modes.
The sticky bit first appeared in V7, and this manual page appeared in section 8.
  Its initial use was to mark sharable executables that were frequently used so
  that they would stay in swap after the process exited. Sharable executables
  were compiled in a special way so their text and read-only data could be
  shared amongst processes. vi(1) and
  sh(1) were such executables. This is
  where the term “sticky” comes from - the program would stick
  around in swap, and it would not have to be fetched again from the file
  system. Of course as long as there was a copy in the swap area, the file was
  marked busy so it could not be overwritten. On V7 this meant that the file
  could not be removed either, because busy executables could not be removed,
  but this restriction was lifted in BSD releases.
To replace such executables was a cumbersome process. One had
    first to remove the sticky bit, then execute the binary so that the copy
    from swap was flushed, overwrite the executable, and finally reset the
    sticky bit.
Later, on SunOS 4, the sticky bit got an additional meaning for
    files that had the bit set and were not executable: read and write
    operations from and to those files would go directly to the disk and bypass
    the buffer cache. This was typically used on swap files for NFS clients on
    an NFS server, so that swap I/O generated by the clients on the servers
    would not evict useful data from the server's buffer cache.