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May I know why there is no such word in many dictionaries ?

It is supposed to be a water mark left after something is being washed.
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Isn't it two words: Water spot?
Comments  
http://www.yourwebassistant.net/glossary/w2.htm#water_spot

I finally got it from the above link. Yeah, it is supposed to be in two words. But I saw it in one in an advertisement. And strange to me that it uses it as a verb too.

http://www.englishdaily626.com/ads-febreze.html

I fail to locate the ads anymore. But I've copied down the sentences and pasted in on the above link.