"I want you should sell to me. I don't say what I'm going to do with the property, and you will not have an iota of responsibility, whatever happens."
—William Dean Howells, The Rise of Silas Lapham, 1885
Iota is the 9th letter of the Greek alphabet, equivalent to our letter i. Latin scholars transcribed the Greek name as iota and jota, and both iota and jot represent the name for the Greek letter in English.
The letter's slightness of shape led to the use of iota to mean "a small amount." It is often used in negative contexts, such as "not one iota of truth."
Jot, incidentally, appears in the phrase every jot and tittle, referring to small details. The word tittle refers to the dot that we place over a lowercase i or j.
(source: Merriam-Webster Online)