Source : Webster's Revised Unabridged Dictionary (1913)

Undercurrent \Un"der*cur`rent\, a.
   Running beneath the surface; hidden. [R.] ``Undercurrent
   woe.'' --Tennyson.

Undercurrent \Un"der*cur`rent\, n.
   1. A current below the surface of water, sometimes flowing in
      a contrary direction to that on the surface. --Totten.

   2. Hence, figuratively, a tendency of feeling, opinion, or
      the like, in a direction contrary to what is publicly
      shown; an unseen influence or tendency; as, a strong
      undercurrent of sentiment in favor of a prisoner.

            All the while there was a busy undercurrent in her.
                                                  --G. Eliot.

Source : WordNet®

undercurrent
     n 1: subdued emotional quality underlying an utterance; implicit
          meaning [syn: {undertone}]
     2: a current below the surface of a fluid [syn: {undertide}]