in (one's) back pocket

in (one's) back pocket

Available to be utilized if and when it is required, as of a secret plan, idea, or advantage. Keep that recording of your boss in your back pocket—you'll need it later if he denies what he said. The defense attorney has kept this eye-witness in her back pocket, just waiting for the right time.
See also: back, pocket
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.