WordNet (r) 3.0 (2006):

plonk
    n 1: a cheap wine of inferior quality
    2: the noise of something dropping (as into liquid)
    v 1: set (something or oneself) down with or as if with a noise;
         "He planked the money on the table"; "He planked himself
         into the sofa" [syn: plank, flump, plonk, plop,
         plunk, plump down, plunk down, plump]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

PLONK
       Please Leave Our Newsgroup, Kid (Usenet, slang)


The Jargon File (version 4.4.7, 29 Dec 2003):

plonk
 excl.,vt.

    [Usenet: possibly influenced by British slang ?plonk? for cheap booze, or
    ?plonker? for someone behaving stupidly (latter is lit. equivalent to
    Yiddish schmuck)] The sound a newbie makes as he falls to the bottom of a
    kill file. While it originated in the newsgroup talk.bizarre, this term
    (usually written ?*plonk*?) is now (1994) widespread on Usenet as a form of
    public ridicule.