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]