Why is it called a toothbrush instead of teethbrush?

"Toothbrush was invented in Kentucky. If it was invented in any other place, they would have named teethbrush"

I got this snippet from a friend of mine. This is one of the regular snippet that's being sent on email that started recently. I did get a chuckle out of this. Immediately, the curiosity in me took me under control and I involuntarily surrendered to my thought process, digging on the Internet for a possible relation between Kentucky and Toothbrush.
First, I began searching on who invented the toothbrush. I got information on how twigs, miswak, roots etc were being used. The Chinese invented the modern toothbrush in 1400s, the first mass-production of toothbrush in England in 1700s. Then I came across a snippet about first US patent for toothbrush was awarded to H.N Wadsworth in 1857.

So there I thought I got a lead. I checked if Wadsworth belonged to Kentucky, no leads. I did read some interesting funny thoughts about why its called a toothbrush and not a teethbrush even though we have teeth.
Few ideas were like "The person who invented it had just one tooth". "People were so unhygienic in older times that they had just one tooth left over or mostly toothless"....and so on.

I still didn't get an idea as to why Kentucky. There were similar statements that I came across on the Internet but this time replaced with Arkansas and then one more with Tennessee.

http://www.drug3k.com/forum/Dental/Why-do-they-call-it-a-toothbrush-if-we-have-many-teeth-7240.htm

"Toothbrush was invented in Arkansas. If it was invented in any other place, they would have named teethbrush"

"Toothbrush was invented in Tennessee. If it was invented in any other place, they would have named teethbrush"

"Toothbrush was invented down south. If it was invented in any other place, they would have named teethbrush"


Now what is common between Kentucky, Arkansas, Tennessee? My curious nature just could not stop and there I go maps.google.com.
I see all these states are in south-east of USA. So those statements were actually a mockery of the intelligence of the people down south? I don't know.

Whatever....after probing..I got the right answer, and here I am putting it in simple words.

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The word is made up of two nouns - "tooth" and "brush"; thus its an compound noun; but the two words are written together. The first word "tooth" describes the second word "brush" and it gives a character to the second word.

The first word "tooth" thus acts as an adjective and becomes an "attribute noun". As it acts as an adjective, it follows the

• rule of NOT PLURALIZING ADJECTIVES.
• Gives importance to the TYPE of element its relating to rather than "NUMBER".

Eg . shoe rack, footrest, toothpaste, nail enamel, hairbrush......

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Hmmm...a bit of gyan here and I am happy that I can sleep without a thing biting my brain.