1. [common] To delete something, usually superfluous, or to abort an operation. “All that nonsense has been flushed.”
2. [Unix/C] To force buffered I/O to disk, as with an fflush(3) call. This is not an abort or deletion as in sense 1, but a demand for early completion!
3. To leave at the end of a day's work (as opposed to leaving for a meal). “I'm going to flush now.” “Time to flush.”
4. To exclude someone from an activity, or to ignore a person.
‘Flush’ was standard ITS terminology for aborting an output operation; one spoke of the text that would have been printed, but was not, as having been flushed. It is speculated that this term arose from a vivid image of flushing unwanted characters by hosing down the internal output buffer, washing the characters away before they could be printed. The Unix/C usage, on the other hand, was propagated by the fflush(3) call in C's standard I/O library (though it is reported to have been in use among BLISS programmers at DEC and on Honeywell and IBM machines as far back as 1965). Unix/C hackers found the ITS usage confusing, and vice versa.