Why are you a plonker?

The idiom, plonk (something/someone) down means

• to slap something down; to plop something down
• to sit or lie down on something in a careless or noisy way
• to leave someone somewhere to do this; Dave plonked the kids in front of the TV and disappeared upstairs.
• to put something down heavily and without taking care: Just plonk the shopping (down) on the table, and come and have a cup of tea. Come in and plonk yourselves (down) (= sit down) anywhere you like.

From these various definitions I can surmise why cheap wine is often called plonk, it's the sound of the bottle slapping down heavily on the table.

But how did we get from that to “a plonker” which basically means a silly or stupid person.

As in

"Why did you do that, you plonker?"

_{Nicholas Lyndhurst who played Rodney Trotter in Only Fools and Horses}

References: FD; plonker "Sir David Jason says an American remake of Only Fools And Horses won't work as there's no word over there for plonker." CDO; plonk

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!'