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.