VERB (3)

1. double in magnitude, extent, or intensity;
- Example: "The enemy redoubled their screaming on the radio"

2. double again;
- Example: "The noise doubled and redoubled"

3. make twice as great or intense;
- Example: "The screaming redoubled"

The Collaborative International Dictionary of English v.0.48:

Redouble \Re*dou"ble\ (r?*d?b"'l), v. t. [Pref. re- + double:
   cf. F. redoubler. Cf. Reduplicate.]
   To double again or repeatedly; to increase by continued or
   repeated additions; to augment greatly; to multiply.
   [1913 Webster]

         So they
         Doubly redoubled strokes upon the foe.   --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Redouble \Re*dou"ble\, v. i.
   To become greatly or repeatedly increased; to be multiplied;
   to be greatly augmented; as, the noise redoubles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

redouble
    v 1: double in magnitude, extent, or intensity; "The enemy
         redoubled their screaming on the radio"
    2: double again; "The noise doubled and redoubled"
    3: make twice as great or intense; "The screaming redoubled"