control-S

from Jargon File (4.4.4, 14 Aug 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.

from The Free On-line Dictionary of Computing (8 July 2008)

control-S
XOFF

   <character> (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)