Why are you a plonker?

The Urban Dictionary suggests plonker is a person habitually drunk on cheap wine , (plonk) and hence someone who is foolish or useless.

I don't think that's right. I believe plonker in this context is a slang term for penis (chiefly used in the term pull someone's plonker, attempt a mild deception). Slang terms for penis are routinely used as terms of abuse, and that is why plonker is used for this purpose.

Edit I've just had a chance to look in Green's Dictionary of Slang. He has three entries for plonker:-

Anything large or substantial (figurative usage of standard English plonk, to hit or strike with a plonking noise). Earliest reference 1861.

Also plonk, the penis, earliest reference c1920 in the phrase pull one's plonker

Also plonk, a general term of abuse, widely popularized by the BBC TV Series Only Fools and Horses, earliest reference 1959.

The origin of plonker is from plonk + -er¹, where plonk is a verb² meaning

To hit or strike [something] with a heavy thud

Although this meaning is now rare.

Plonker itself has a few meanings, including "something large and substantial of its kind", "penis" and "a foolish, inept, or contemptible person"¹.

The first use of plonker to mean "a foolish, inept, or contemptible person" is attested in the OED to be an episode of Only Fools and Horses (in 1981)¹. I would imagine that this use is actually related to the use of plonk to mean "To set or drop (a thing) in position heavily or clumsily"² as inept people are wont to.

However, as Brian Hooper points out, plonker can mean penis¹. The OED gives this example usage:

to pull one's plonker : to masturbate

This seems just as likely the origin, for the reasons Brian mentions, although not before the BBC watershed.

1. "plonker, n.". OED Online. June 2013. Oxford University Press. http://www.oed.com/view/Entry/145905?redirectedFrom=plonker (accessed July 26, 2013).

2. "plonk, v.". OED Online. June 2013. Oxford University Press. http://www.oed.com/view/Entry/145903 (accessed July 26, 2013).

My understanding of the definition of 'plonker' is quite different! It stems from the very popular UK television series of the 1980s called Only Fools And Horses, set in London's Peckham.

Writer John Sullivan used a number of sound-alike words to substitute for non-permissible rude ones and 'plonker' was one of these. Use your imagination for what he meant when lead character, Del-Boy said to his brother 'don't be a plonker, Rodney!'

There are some references on the web to plonk as the lowest ranking person in the Royal Air Force (RAF), which apparently is an Aircraftman 2nd class, sometimes referred to as an AC plonk, and another for plonk as a slang term for mud, coined in the trenches during the Great War, so its origins could lie in the notion of a person of lowest rank or status, or someone down in the mud.