The Collaborative International Dictionary of English v.0.48:

fleet \fleet\ (fl[=e]t), v. i. [imp. & p. p. fleeted; p. pr. &
   vb. n. fleeting.] [OE. fleten, fleoten, to swim, AS.
   fle['o]tan to swim, float; akin to D. vlieten to flow, OS.
   fliotan, OHG. fliozzan, G. fliessen, Icel. flj[=o]ta to
   float, flow, Sw. flyta, D. flyde, L. pluere to rain, Gr.
   plei^n to sail, swim, float, Skr. plu to swim, sail.
   [root]84. Cf. Fleet, n. & a., Float, Pluvial, Flow.]
   1. To sail; to float. [Obs.]
      [1913 Webster]

            And in frail wood on Adrian Gulf doth fleet.
                                                  --Spenser.
      [1913 Webster]

   2. To fly swiftly; to pass over quickly; to hasten; to flit
      as a light substance.
      [1913 Webster]

            All the unaccomplished works of Nature's hand, . . .
            Dissolved on earth, fleet hither.     --Milton.
      [1913 Webster]

   3. (Naut.) To slip on the whelps or the barrel of a capstan
      or windlass; -- said of a cable or hawser.
      [1913 Webster]

   4. (Naut.) To move or change in position; -- said of persons;
      as, the crew fleeted aft.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Fleeting \Fleet"ing\, a.
   Passing swiftly away; not durable; transient; transitory; as,
   the fleeting hours or moments.

   Syn: Evanescent; ephemeral. See Transient.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fleeting
    adj 1: lasting for a markedly brief time; "a fleeting glance";
           "fugitive hours"; "rapid momentaneous association of
           things that meet and pass"; "a momentary glimpse" [syn:
           fleeting, fugitive, momentaneous, momentary]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "fleeting":
   abrupt, blunt, brief, brittle, brusque, capricious, changeable,
   compendious, corruptible, crusty, curt, deciduous, disappearing,
   dissolving, dying, ephemeral, evanescent, evaporating, fading,
   fickle, flitting, fly-by-night, flying, fragile, frail, fugacious,
   fugitive, gruff, impermanent, impetuous, impulsive, inconstant,
   insubstantial, laconic, melting, momentary, mortal, mutable,
   nondurable, nonpermanent, passing, perishable, short,
   short and sweet, short-lived, snippety, snippy, succinct, temporal,
   temporary, terse, transient, transitive, transitory, undurable,
   unenduring, unstable, vanishing, volatile