alternative meaning of "pickle"

I've been reading a book by Sue Grafton titled "W is for Wasted" and have encountered a phrase "to pickle the average adult"

I have looked up the meaning of the verb and that's what the Oxford Dictionary provided me with:

1) preserve (food or other perishable items) in vinegar or brine

2) immerse (a metal object) in an acid or other chemical solution for cleaning

However, I still don't understand the meaning of the phrase.

he'd enjoyed alcohol in sufficient quantities to pickle the average adult

I guess it's a North American slang or something.

It is slang, and somewhat relies on one knowing that you can preserve things in ethanol, which is the pure form of alcohol (what actually happens is that alcohol is broken down by the liver and converted to acetaldehyde, which is a close relative of formaldehyde (which I'm sure you know is used in embalming). but that's not so much a useful part of the answer as some interesting biochemistry waffle).

In this context, "pickle" is being used in its definition of

preserve (food or other perishable items) in vinegar or brine

and a link is being made between the preservation of food with vinegar, and a human consuming so much alcohol that the resulting acetaldehyde formation would preserve them the same way.

I should point out that the way your sample phrase is worded would imply exaggeration of the amount consumed; it's not literally saying that they've consumed a sufficient quantity to cause that. (as WS2 pointed out, "being pickled" is used as hyperbole for "intoxicated")

To Pickle does usually mean to preserve with vinegar or acid. But alcohol is a preservative too, and the term pickling can be used, certainly in British English, to refer to the process of preserving in alcohol.

Pickling usually refers to the process of fermenting a food in salty brine. However, another kind of food preservation can involve simply soaking food in a solution that keeps it from spoiling. Alcohol, including vodka, is one liquid you can use. [From eHow's article on the subject of pickling in vodka]

So the usage above is (somewhat humorously) suggesting that the person in question had drunk so much alcohol he could have preserved himself permanently in it.

As a reader of Sue Grafton I feel certain that the usage she intended you to understand is: "drunk (on alcohol)." "Pickled" is a common slang term (or was) for "drunk:"

2. Slang. drunk; intoxicated.

The specific sentence you reference is meant to convey that the person described has a prodigious appetite for alcohol, and consumes it in quantities the average person would find debilitating.

The references to using alcohol as a food preservative are best understood as the basis for this slang expression.

Far from an average person, but Lord Nelson's body was pickled in brandy to preserve it on the return trip to England, following his death at the battle of Trafalgar. When the cask was opened it contained his body, but no brandy. Sailor's had drained the liquid content and drank it. So to be pickled in alcohol has a literal, historical context. Hence the quote is hyperbole, as for a single person to consume the amount of alcohol required to literally pickle a person, in a reasonable period of time, would be impossible.