As in expressions like ‘not worth an iota’ or ‘not one iota of truth’ means worthless or totally untruthful. According to the OED, this usage of iota dates from the late 16th/early 17th century, but goes on to add that it is a straight borrowing from the Greek iota, which when written as ‘i’, is the smallest letter in the Greek alphabet and hence the smallest or least of anything. This is odd because the OED maintains the word ‘jot’ derives in turn from ‘iota’, but dates the word jot from 1499, which is slightly earlier than the dates given for ‘iota’. Despite this anomaly, there is no question that ‘iota’ and ‘jot’ are linked etymologically. See also jot.