The Free On-line Dictionary of Computing (19 January 2023):

control-S
XOFF

    (XOFF, DC3, Device Control 3, ASCII code 19).
   A character used in software handshaking to temporarily
   suspend output until a control-Q is received.

   [Jargon File]

   (1996-06-28)


The Jargon File (version 4.4.7, 29 Dec 2003):

control-S
 vi.

    “Stop talking for a second.” From the ASCII DC3 or XOFF character (the
    pronunciation /X-of/ is therefore also used). Control-S differs from 
    control-O in that the person is asked to stop talking (perhaps because you
    are on the phone) but will be allowed to continue when you're ready to
    listen to him — as opposed to control-O, which has more of the meaning of “
    Shut up.” Considered silly.