The Collaborative International Dictionary of English v.0.48:

Fugacious \Fu*ga"cious\, a. [L. fugax, fugacis, from fugere: cf.
   F. fugace. See Fugitive.]
   1. Flying, or disposed to fly; fleeing away; lasting but a
      short time; volatile.
      [1913 Webster]

            Much of its possessions is so hid, so fugacious, and
            of so uncertain purchase.             --Jer. Taylor.
      [1913 Webster]

   2. (Biol.) Fleeting; lasting but a short time; -- applied
      particularly to organs or parts which are short-lived as
      compared with the life of the individual.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "fugacious":
   brittle, capricious, changeable, corruptible, deciduous, dying,
   ephemeral, evanescent, fading, fickle, fleeting, flitting,
   fly-by-night, flying, fragile, frail, fugitive, impermanent,
   impetuous, impulsive, inconstant, insubstantial, momentary, mortal,
   mutable, nondurable, nonpermanent, passing, perishable,
   short-lived, temporal, temporary, transient, transitive,
   transitory, undurable, unenduring, unstable, volatile