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]WordNet (r) 3.0 (2006):
undertide n 1: a current below the surface of a fluid [syn: undertide, undercurrent]