keep so or sth back meaning

[American slang]
to hold someone or something in reserve.
  Keep back some of the food for an emergency.
  We are keeping Karen back until the other players have exhausted themselves.

• keep sth in the back of so's mind:    [American slang]Go ...
• keep so, sth, or an animal back:    [American slang]to ...
• keep off (of) so's back:    [American slang]In ...