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.


The Free On-line Dictionary of Computing (30 December 2018):

control-S
XOFF

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

   [Jargon File]

   (1996-06-28)