The Collaborative International Dictionary of English v.0.48:

Frustrate \Frus"trate\, v. t. [imp. & p. p. Frustrated; p. pr.
   & vb. n. Frustrating.]
   1. To bring to nothing; to prevent from attaining a purpose;
      to disappoint; to defeat; to baffle; as, to frustrate a
      plan, design, or attempt; to frustrate the will or
      purpose.
      [1913 Webster]

            Shall the adversary thus obtain
            His end and frustrate thine ?         --Milton.
      [1913 Webster]

   2. To make null; to nullifly; to render invalid or of no
      effect; as, to frustrate a conveyance or deed.

   Syn: To balk; thwart; foil; baffle; defeat.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

frustrating \frustrating\ n.
   an act of hindering someone's plans or efforts.

   Syn: frustration, thwarting, foiling.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

frustrating \frustrating\ adj.
   1. preventing realization or attainment of a desire; causing
      frustration.

   Syn: frustrative, thwarting.
        [WordNet 1.5]

   2. discouraging. Opposite of encouraging.

   Syn: disappointing.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

frustrating
    adj 1: discouraging by hindering
    2: preventing realization or attainment of a desire [syn:
       frustrating, frustrative, thwarting]