1. [n]  the noise of something dropping (as into liquid)
2. [n]  (British and Australian) a cheap wine of inferior quality
3. [v]  set (something or oneself) down; "He planked the money on the table"; "He planked himself into the sofa"

(Possibly influenced by British slang "plonk" for cheap booze, or "plonker" for someone behaving stupidly; usually written "*plonk*") The sound a newbie makes as he falls to the bottom of a kill file. While this term originated in the Usenet newsgroup news:talk.bizarre, by 1994 it was widespread on Usenet and mailing lists as a form of public ridicule.

Another theory is that it is an acronym for "Person with Little Or No Knowledge".