These words were all over the UK media a few years ago describing questionable police techniques during demonstrations in London and were new to me.
Last Sunday’s Observer, describing Wall Street protests, says

NYPD officers strung orange netting across the streets to trap groups of protesters, a tactic described by some of them as “kettling” – a term more commonly used by critics of a similar tactic deployed by police in London to contain potentially violent demonstrations there.

It seems to have crossed the pond if it wasn’t already there.

Wordnik has a dodgy citation (see Wikipedia quote below)

“The phrase kettling is a translation from the german word used to describe the police tactic in the 60’s known as the Hamburger Kessel.”
The Led

and a poster adds

* john
* Apr 15, 2009

Waddington helped to develop “kettling,” where police enclose protesters in a confined space, a tactic that replaced the use of horses or crowd charges by lines of baton-wielding officers to disperse demonstrations.
The New York Times, Technology Advances Put Police Behavior In Focus, April 15, 2009

Who is Waddington?

Wikipedia under Hamburg Police has

Hamburg pocket

On 8 June 1986, the Hamburg Police closed in on 861 protesters and contained them in the open area of the Heiligengeistfeld for 13 hours. The demonstrators were held without food, water and toilets. Demonstrations against the use of nuclear power developed in the aftermath of the Chernobyl disaster. On the day before, groups of protestors, on their way to the Brokdorf Nuclear Power Plant, were stopped by the police. On Sunday, 8 June, several people of the anti-nuclear movement wanted to protest against the police actions. The Hamburger Kessel (lit. Hamburg pocket, the word Kessel can also be translated as kettle.) were sentenced legal wrong, by the Hamburg regional court, and all involved were adjudged a solatium of DM200. The 4 police leaders of the Hamburg pocket were declared guilty of deprivation of personal freedom, but only admonishment and had to pay a fine.

Pocket makes more sense than kettle but how did it suddenly pop up in British English (as a probable eggcorn) after lying dormant for over 20 years and has it gone beyond the NYPD in the States? However, I cannot find a definition of German Kessel meaning pocket anywhere online. The guy who wrote the Wikipedia excerpt clearly doesn’t have a problem with English except for minor errors after ‘legal wrong’. Maybe it is German slang.

(Kessel also means boiler in German as in this
machine translation).

Who is Waddington?

See: Professor defends ‘kettling’ technique used at G20 protests

Apr 21 2009 by Prof Peter Waddington, Birmingham Post

http://www.birminghampost.net/comment/birmingham-columnists/agenda/2009/04/21/professor-defends-kettling-technique-used-at-g20-protests-65233-23429856/

Obviously German ‘Kessel’ has a figurative meaning of people or animals herded together. If you google on kessel+ zusammengetrieben (herded together), you will find many instances of that use (if you can figure out at least some German).

The image seems pretty clear to me. Think of a cauldron with everything thrown in.

Dutch ‘heksenketel’ (cauldron) is also often used to describe a great mass of people.

What the proper English word for that same image would be, I wouldn’t know.

I guess (and it is just that) that kettling appeared in the UK because the UK police needed a different method of crowd control so looked for foreign solutions to the same problem.  They then adopted the tactic and the name from the German police, hence the 20-year gap.  There’s no verb “kettle” in OED and “kettling” appears only as an alternative form of chitterling, small fried pieces of the pig, which is vaguely connected to German “kutteln”.

Thanks, the figurative use in German and Dutch is now clear.
British police probably misheard Kessel as kettle unless the Dutch cauldron analogy was familiar to them (but how?)
There is no credence to the ‘pocket’ translation.

venomousbede - 27 September 2011 08:26 AM

Thanks, the figurative use in German and Dutch is now clear.
British police probably misheard Kessel as kettle unless the Dutch cauldron analogy was familiar to them (but how?)
There is no credence to the ‘pocket’ translation.

I think the British cops just translated the term as the Dutch and German versions are cognate.

Certainly not a case of a different kettle of fish (you groaned).

I am not convinced that monoglot British coppers would go to so much trouble. European police forces liaise often (in English so the Brits won’t feel left out) and it could have come from a conference on riot or crowd control with British cops saying OK we’ll call it something like what they do.
No doubt the OED will be on this eventually.

venomousbede - 27 September 2011 08:26 AM

Thanks, the figurative use in German and Dutch is now clear.
British police probably misheard Kessel as kettle unless the Dutch cauldron analogy was familiar to them (but how?)
There is no credence to the ‘pocket’ translation.

Kessel in German can refer to an encircled military force or the space it occupies - the German 6th Army near the end of the Battle of Stalingrad in WW2, for example.  Though it’s been a few years since I’ve read it, I’m fairly certain Anthony Beevor used this word frequently in his (English-language) book about the battle.  In this context, pocket is an English equivalent.

NotThatGuy - 07 October 2011 01:29 PM

In this context, pocket is an English equivalent.

Doesn’t really have that possibly unconsciously ironic overtone of pressure building up till something has to give though, does it?