The Collaborative International Dictionary of English v.0.48:

Sleuth \Sleuth\, n. [Icel. sl[=o][eth]. See Slot a track.]
   The track of man or beast as followed by the scent. [Scot.]
   --Halliwell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sleuth
    n 1: a detective who follows a trail [syn: sleuth,
         sleuthhound]
    v 1: watch, observe, or inquire secretly [syn: spy, stag,
         snoop, sleuth]