The Collaborative International Dictionary of English v.0.48:

Underplay \Un`der*play"\, v. i.
   1. To play in a subordinate, or in an inferior manner; to
      underact a part.
      [1913 Webster]

   2. (Card Playing) To play a low card when holding a high one,
      in the hope of a future advantage.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underplay \Un"der*play`\, n. (Card Playing)
   The act of underplaying.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underplay
    v 1: act (a role) with great restraint [syn: underact,
         underplay] [ant: ham, ham it up, overact,
         overplay]
    2: play a card lower than (a held high card)