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]

Moby Thesaurus II by Grady Ward, 1.0:

163 Moby Thesaurus words for "undercurrent":
   affect, affection, affluence, afflux, affluxion, air current,
   allegory, allusion, ambiance, ambience, arcane meaning, assumption,
   atmosphere, aura, bucking, challenge, coloration, concourse,
   confluence, conflux, connotation, contention, contradiction,
   contraposition, contravention, contraversion, coral heads,
   counteraction, counterbalance, counterblast, countercurrent,
   counterforce, counterinfluence, counterpoise, counterpressure,
   counterweight, counterworking, course, crosscurrent, crossing,
   current, current of air, defluxion, denial, downdraft, downflow,
   downpour, draft, drift, driftage, emotion, emotional charge,
   emotional shade, experience, fall wind, feeling, feeling tone,
   flavor, flow, flow of air, flowing, fluency, flux, following wind,
   foreboding, foul wind, gush, gut reaction, head wind, heartthrob,
   hint, implication, implied meaning, import, impression,
   impugnation, impugnment, indraft, inference, inflow, inhalation,
   innuendo, inrush, inspiration, intimation, ironbound coast,
   ironic suggestion, jetstream, katabatic wind, ledges, lee shore,
   meaning, metaphorical sense, mill run, millrace, monsoon,
   movement of air, murmur, negation, nuance, occult meaning, onrush,
   onward course, opposing, opposition, opposure, oppugnation,
   outflow, overtone, passion, pitfall, presentiment, presumption,
   presupposition, profound sense, quicksands, race, reaction,
   rebutment, rebuttal, refusal, rejection, resistance, response,
   rockbound coast, rocks, run, rush, sandbank, sandbar, sands,
   sensation, sense, sentiment, set, shallows, shoals, spate,
   standing against, stream, stream of air, subsense,
   subsidiary sense, suggestion, supposition, surge, symbolism,
   tail wind, tendency, tenor, tide, tinge, touch, traversal, trend,
   undermeaning, undertone, undertow, updraft, vibes, vibrations,
   water flow, wind