An iota is the smallest possible amount. Not one iota, not an iota of evidence, not an iota of difference.
It appears only in the negative. Iota is the smallest letter of the Greek alphabet, and the phrase comes from a line about not one jot changing in the law — jot is the same word arriving through Latin, so the two are doublets.
1nouniotaThe smallest possible amount.
“There isn't an iota of truth in what he told you.”
iota is an amount.
Greek iota is the smallest letter of the alphabet. Jot is the same word by another road.
Proto-Semitic
*yad- — hand
Ancient Greek
ἰῶτα
English, today
iota
From Ancient Greek ἰῶτα, ultimately from Proto-Semitic yad- (“hand”). a doublet of jot and yodh. Sense “jot, small quantity” in reference to a phrase in the New Testament: ”one iote or one title, shall in no wise passe from the law”, iota being the smallest letter of the Greek alphabet.
Etymology from Wiktionary, CC BY-SA