undertide

from WordNet (r) 3.0 (2006)

undertide
    n 1: a current below the surface of a fluid [syn: {undertide},
         {undercurrent}]

from The Collaborative International Dictionary of English v.0.48

Undertide \Un"der*tide`\, Undertime \Un"der*time`\, n. [Under +
   {tide}, time. Cf. {Undern}.]
   The under or after part of the day; undermeal; evening.
   [Obs.]
   [1913 Webster]

         He, coming home at undertime, there found
         The fairest creature that he ever saw.   --Spenser.
   [1913 Webster]