"many"
沢山
marsh-mountain

If you can, please post the site.

There is no etymology. They used Kanji which would be spoken in the same way as たくさん. The Word for this kind of usage is ATEJI.

Here 2 word, where they did the same thing:
面白い　　おもしろい
美味しい　　おいしい
寿司　　　　すし
独逸 ドイツ

ATEJI: kanji used as a phonetic symbol, instead of for the meaning; phonetic-equivalent character; substitute character

More information under:
http://en.wikipedia.org/wiki/Ateji

Here is 「沢山」の語源 on yahoo answers japan:
http://detail.chiebukuro.yahoo.co.jp/qa/...1019150715

Quote:The word "沢山" is now read "たくさん" but originally, in the Edo era, it was read as "さわやま" (the word was mostly used by women)

さわやま had the same meaning as today's たくさん. さわやま could be written in kanji in two ways, 沢山 and 多山. The word たくさん has the meaning "extremeness in quantity and degree" and perhaps this came from "多山（さわやま）", because it is like 多い山, a lot of mountains.

Gradually 沢山（さわやま） became widespread. Today it is read using the on-reading, たくさん.

This is my translation so there might be mistakes.

Also, I think 面白い might also not be ateji.
おも‐しろ・い【面白い】
［形］［文］おもしろ・し［ク］《もと、目の前が明るくなる感じをいった語》......
- 大辞林

Thanks, that's interesting.

@teskal: There is no such thing as "no etymology". Every word came from somewhere, even if the origins are lost to history.
@Amset: What you wrote isn't accurate. Or rather, what is written on Chiebukuro isn't.

さわやま is Edo period (1600s-1800s) slang, while たくさん dates at least to the 1200s (Heike Monogatari). Edo slang is usually based on intentionally misreading kanji, reversing morpheme order, etc.

Heikei Monogatari (written sometime in the 1200s), volume 8 Wrote:かまくらいでのしゆくよりも、あふみのくにかがみのしゆくにいたるまで、しゆくじゆくにじつこくづつのよねをおかれたりければ、たくさんなるによつて、せぎやうにひきけるとぞきこえし。

The etymology of たくさん is listed as unknown (語原未詳) in dictionaries, which isn't surprising since it's an old word of Japanese (和語), not Chinese (漢語), origin. The characters for it are indeed ateji (借字).

It looks like whoever replied on that Chiebukuro thread just looked it up in Kojien, didn't fully read or understand the definition, and then made some guesses. Folk etymology sucks.