un·der·tow /-ˌto/
  從岸邊退回去的波浪;退波
  Un·der·tow n. Naut. The current that sets seaward near the bottom when waves are breaking upon the shore.
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  undertow
       n 1: an inclination contrary to the strongest or prevailing
            feeling; "his account had a poignant undertow of regret"
       2: the seaward undercurrent created after waves have broken on
          the shore [syn: sea puss, sea-puss, sea purse, sea-purse,
           sea-poose]