What does "sock puppeting" mean?

I found a Meta Stack Overflow post in whose answer Servy says:

it's sock puppeting and is a very serious violation

"Sock puppeting" is a picture. But why a sock? Is it like playing with your own sock?

Simply, a sock-puppet is a puppet which has been created using a sock.

In this context, a sock-puppet is one person pretending to be another. What you see is the puppet but the actions and voice are those of the person controlling the puppet.

On messageboards and websites a sock puppet is the online alter-ego of somebody else.

Urban Dictionary

The most popular definition on Urban Dictionary is also the earliest, from 2003:

1. sock puppet

An account made on an internet message board, by a person who already has an account, for the purpose of posting more-or-less anonymously.

by Ian Maxwell Mar 14, 2003

Oxford English Dictionary

The OED doesn't have this meaning, but has two earlier meanings that shows the development:

sock puppet n. orig. U.S. a simple puppet made from a sock that is fitted over and moved by the hand and fingers; (also fig.) a person whose actions are controlled by another; a minion.

The first hand puppet use is:

1959 Gettysburg (Pa.) Times 24 Sept. 12 (advt.) Shari Lewis Sock Puppet Reg. $1.39.

The first minion use is:

2000 U.S. News & World Rep. 27 Mar. 22/1 Jennifer Brand, a 24-year-old student who backed President Clinton in 1996, called Gore ‘a sock puppet’ and Bush ‘a wimp’.

A similar figurative use can be found in the older term, puppet government, which can be traced back to puppet King used in 1611.

Jargon File

Here's the definition in Jargon File version 4.1.0, 12th March 1999 (the first version "since the last paper revision of The New Hacker's Dictionary"):

:sock puppet: n. [USENET: from the act of placing a sock over your hand and talking to it and pretending it's talking back] In Usenet parlance, a {pseudo} through which the puppeteer posts follow-ups to their own original message to give the appearance that a number of people support the views held in the original message.

Usenet

Wikipedia says:

The term "sockpuppet" was used as early as July 9, 1993, but did not become common in USENET groups until 1996.

Here's the 1993 in bit.listserv.fnord-l:

Fuck both you clowns!
~~~~
I suppose I could save everyone the suspense by just saying, "Fuck you, clown (singular)", though. Everyone knows they're seeing two when there's only one. I happen to know for a fact that one is merely the sock puppet manifestation of the other's demented and sadly listing psyche.

Let's delve further into Usenet to see where the fake account meaning comes from.

rec.gambling.blackjack

It was used extensively in rec.gambling.blackjack from September 1996 onwards in hundreds of posts about one person's gambling method and the numerous sock puppets who backed him up. These sock puppets were referred to as separate individuals (December 1996):

Whether you are a hired sock puppet or an independent sock puppet is rather immaterial. None of the sock puppets have ever denied our claim that the "hired typist" that you share is none other than Doug Grant himself.

But some suggestions were made it was the same person, for example (September 1996):

Doogie has 2 hands, ergo room for 2 sock puppets. If he could figure out how to put a sock puppet on his foot, he could play bridge.

And in September 1996:

The problem is that Doug Grant has a different agenda. He's still fighting an 11 year old battle with Snyder and using fake people to help him wage this war. By the way, did Doug pay for your account. Give him a call, he's paid for several others. He even gets his sock puppets their own accounts.

And in September 1997:

What is it - Sock Puppets? Does Doogie hire the illiterate, or are you going to go back to that "shared typist" excuse?

By February 1998, sock puppet had both meanings of a minion and of a fake account: here's "Sock puppet defined":

So, you post what you are told to post, when you are told to post, and you stop posting when you are told to stop.

Pretty simple definition of a "sock puppet", eh?

Sock Puppet (n.)

an alternate, ficticious net-persona adopted by a net.kook
to make it appear that there are others who agree with him.

"On the internet, no one knows you're a dog."

Pete Moss (so far as you know)

@Ste's answer is correct and should be the one accepted, but I wanted to add that most messageboards and websites forbid sockpuppets and will sometimes terminate not only the sockpuppet but also the original account of the user who created the sockpuppet in the first place, although sockpuppets are not necessarily created for nefarious purposes. A friend of mine created a sockpuppet on a particular message board in order to argue with himself and create a better experience for everyone on the board, or at least that was his stated intent. On the other hand, a guy that was working to defeat me in an "edit war" on an article on Wikipedia created two sockpuppets to create a false sense of consensus around his point of view. He and his sockpuppets got sent to outer darkness once this was discovered.

It can be more subtle. Two or more people can contrive to create individual accounts that are in fact associated with different people, and still be effectively sockpuppeting, if the accounts were created in order to collude towards a common goal. An example in literature can be found in Orson Scott Card's novel Ender's Game, where Andrew "Ender" Wiggins' brother Peter and sister Valentine post what become influential essays of political discourse as "Locke" and "Demosthenes", respectively, in order to influence public policy. They come across as independent anonymous individuals, but are in fact complicit with each other towards a goal, leading eventually to Peter becoming the effective ruler of Earth.