The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
      [1913 Webster]

            All the while there was a busy undercurrent in her.
                                                  --G. Eliot.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Undercurrent \Un"der*cur`rent\, a.
   Running beneath the surface; hidden. [R.] "Undercurrent woe."
   --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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