Source : Webster's Revised Unabridged Dictionary (1913)

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.]

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

Source : WordNet®

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