"Not one iota" , how much is an iota?

[Loosehead]

... and why this particular greek letter? Anyone help with the origin of this phrase?

[Quizmonster]

A jot - from the Greek letter �iota' = �i' - means the least part of any writing. It can be applied to dots, strokes, Spanish tildes etc. A tittle is defined as a small stroke or point in writing. The two words are, effectively, synonyms, even though they are usually now combined in the phrase "not a jot or tittle".
In the same way, 'not one iota', therefore, means 'not the very least little bit'.

[rojash]

... and why this particular greek letter?
Well, it is the smallest letter in the Greek alphabet, but there is an interesting alternative theory, here:

http://www.randomhouse.com/wotd/index.pperl?da te=20000223

[Quizmonster]

Please note, Loosehead that the link Rojash provides makes it clear that the supposedly 'alternative theory' doesn't really hold water! It says (quote)...
"However, I haven't found any indication that this controversy has anything to do with the meaning of iota." (My italics)
The Oxford English Dictionary, for example, doesnt give it house-room.