The Collaborative International Dictionary of English v.0.48:

Flunk \Flunk\ (fl[u^][ng]k), v. i. [imp. & p. p. Flunked
   (fl[u^][ng]kt); p. pr. & vb. n. Flunking.] [Cf. Funk.]
   To fail, as on a lesson; to back out, as from an undertaking,
   through fear.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flunk \Flunk\, v. t.
   1. To fail in; to fail to pass (a test, examination, or
      course of study). [Colloq. U.S.]
      [1913 Webster +PJC]

   2. To shirk, as a task or duty.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flunk \Flunk\, n.
   A failure or backing out; specifically (College cant), a
   total failure in a recitation. [U.S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

flunk
    n 1: failure to reach a minimum required performance; "his
         failing the course led to his disqualification"; "he got
         two flunks on his report" [syn: failing, flunk] [ant:
         pass, passing, qualifying]
    v 1: fail to get a passing grade; "She studied hard but failed
         nevertheless"; "Did I fail the test?" [syn: fail,
         flunk, bomb, flush it] [ant: make it, pass]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "flunk":
   be found wanting, be unsuccessful, bollix, botch, bungle, bust,
   come to grief, fail, fail of success, fiasco, flunk out, foozle,
   go bankrupt, hash, labor in vain, lose, mess, muddle, not come off,
   not pass, not work, wash out, washout