plonker

   Applied in English dialectal use to anything large or substantial since the 1860s. By the 1960s, in working-class London vernacular, the word referred to a man’s private part, with no special reference to size. The word by then was also being used as a term of abuse, especially for a man, and some speakers may have seen it as a synonym of ‘prick’.

   In the 1980s, thanks to the use of the word in popular television series such as Only Fools and Horses and Minder, the term is far more widely known and used. Many who now use it have probably taken it to mean little more than ‘stupid person’, and the anatomical reference is not widely enough known to cause general offence to television audiences, as the use of ‘you prick’ almost certainly would.

   There is an American term ‘plonk’ which can be applied to an obnoxious person, according to Chapman’s Dictionary of American Slang, but it does not often appear in literary sources. The Australians have ‘plonko’, but this is used of a person addicted to plonk, cheap inferior wine. ‘Plonk’ in that sense is derived from French vin blanc.

   Many native-speakers, hearing ‘plonker’ used as an apparently mild insult, must assume that the word refers to someone who plonks things down in a clumsy way, an ungainly oaf.