The Collaborative International Dictionary of English v.0.48:

off-line \off-line\ adj.
   1. (Computers) Not connected; -- of computers or computer
      peripherals normally connected or intended to be connected
      to other computers by a communications line; as, we can't
      print the document because the printer is off-line.
      [Narrower terms: disconnected, not ready, off;
      unconnected ] {on-line

   Syn: offline, off line(predicate).
        [WordNet 1.5]

   2. Hence: (fig.) Outside of or after a meeting or formal
      discussion; as, we can discuss the details off-line; -- a
      term used at meetings and conferences to suggest
      postponing detailed discussion of a topic so as not to
      occupy the time of a large group most of whom may not be
      interested.
      [PJC]

WordNet (r) 3.0 (2006):

off-line
    adj 1: not on a regular route of a transportation system; "an
           off-line ticket office" [ant: on-line, online]
    2: not connected to a computer network; "off-line resources"
       [ant: on-line, online]

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

off-line

    (Or "offline")

   1. Not directly connected to the computer (e.g., an off-line
   tape drive), or with connection suspended ("take the
   printer off-line").

   Contrast background, on-line.

   2. Not now or not here.  "Let's take this discussion
   off-line."  Specifically used on Usenet to suggest that a
   discussion be moved off a public newsgroup to e-mail.

   See also off-line world.

   [Jargon File]

   (1996-02-02)