In England, plonk can also be used as a verb. To plonk something is to set it down without care, eg;
"Kevin plonked his pint down on the bar, thus spilling most of the contents."
It is thought that this use comes either from to plank, which meant to lay money down on the table as part of a wager, or as Oolong has discovered, possibly originated in the late 19th Century as part of a local dialect and should be compared with plunk.
Another possible derivative used in the UK is plonker