(verb) To modify, alter, change or touch up something.

The act of extreme masturbation. You must "knock one out" whilst in close proximity to any of the following; Your mum, a nun, your boss, a member of parliament, George Michael. A person with capabilities to act upon catching you mid self-abuse obvisouly ups the ante. Ejaculation must be reached before your danger wank target comes (no pun intended) to investigate. The higher the chances of being discovered with one's pants down, pulling one's war face is obviously where the danger comes from. The more danger involved the harder (or softer) it is to complete the task in hand (snigger). The more dangerous the better. The chance of being arrested, pummeled by an angry father or having your hand severed by an arab's sabre means that you are a pro "Danger wanker."

To puke from drinking too much. Other drugs may be involved but it is a strictly alcohol based stunt.

It is an act that should be embraced totally and appreciated by all. If it is only a mild fettle, as in you puke a little but don't mong, then it is recomended you keep on drinking.

In the event of someone fettling, it is a courtesy to then sing "For he's a jolly good fettle" to the tune of "He's a jolly good fellow."

Fettles usually lead to hard monging, passing out and periods of forgetfulness and general confusion.

Fettles aproach differently for different people.
Some come sudden and unexpected, most ammusing if a person has to improvise what they will fettle into. Favourate choices for these are sinks or buckets.

However, the worst kind of fettle will start with a simple queazy feeling in your stomache. The feeling will grow and before too long it will be time and you will know when it is time.
If people are not too drunk to move at this stage they will usually make a crawl for an out door spot out of the path or more traditionally a toilet.
When outdoors, bushes, bridges and rivers, for the daring, are good choices for fettle spots, however this choice lacks a mark that would be made by a straight out fettle in a public place that you would later be able to look at and say "I did that"

The effects of fettling whilst not being able to move are disasterous.
Don't let people pass out in sacred rooms if you fear a fettle. Move them outside, to a toilet or stick them in a bath to bathe in their own filth, those bastads.

In extreme cases, be willing to run ahead of fettlers as they may be lacking the ability to lift toilet seats and open doors.

to fettle is to perform a sexual act on somebody.

Someone who pussies out of something.

To fall into a very sleepy or sleeping state, a wasted kind of state, often after smoking too much.

to fuck.
to fuck an ugly girl.

It comes from the steel industry originaly, as it means 'to scrape the slag out of furnaces'.