Originally it was English slang ..

Posted by remi on Tuesday, 22. September 2009 at 12:59 Bali Time:

In Reply to: Hi Wally what does plonker mean? posted by fordfamilyinc on Tuesday, 22. September 2009 at 10:45 Bali Time:

.. for a condom or a p#nis, but I think these days it just a general mildly disparaging term ..