I've always been under the impression it refered to a "T square," a drafting tool for making perpendicular lines. I'm not 100% sure, but that's what I've always heard.
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I find the answer below (in bold) to be believable:
Obviously, Model T comes from the renown car the Ford Model T. But in a phrase 'It's the Model T of...' what exactly might that imply?
: And is it linked to 'Followed to a T' and 'Prefect to a T'?
Here's what I have found out about 'Fit to a T'
If something "fits to a T" then it's perfect for its purpose. The allusion here is said to be with a T square. This piece of apparatus is so accurate that a precise right angle fits it perfectly.
However neat this suggestion is, there is another possible origin, based on the fact that the saying was in use in the 17th century, before the T square was invented. This one suggests that the T stands for "Title", a minute and precisely positioned pen stroke or printer's mark. A tiny brushstroke was all that distinguished the Hebrew letter "dalet" from "resh". "Title" was the word chosen by Wycliffe to translate references to this tiny difference in his version of the New Testament. Thus the mark was perfectly suited to its task.
We've had this discussion before, and some have expressed scepticism about the history of the T sqaure, as not being that old.
Source:phrases.org.uk/bulletin_board/12/messages/404.html
Posted by James Briggs on December 10, 2001
In Reply to: 402.htmlposted by Indefual on December 10, 2001
This is not an idiom - it means exactly what it says. You should stay fit and healthy.
It is just an idiom and has no history.
Meaning he will help you out.
Palestinian and Persian
food
The origin of the idiom 'fudging the truth' is unclear, but it likely comes from the verb 'fudge,' which originally meant to fit or adjust something improperly. Over time, 'fudging' came to imply manipulating or distorting the truth in a deceitful manner.
To be exposed
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This is not an idiom - it means exactly what it says. You should stay fit and healthy.
Origin "up a storm"
That's not an idiom. It means just what it looks like -- something is fit for you to eat.
No
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affrica (iraq
RELAX
It is just an idiom and has no history.
grab a bite