The iota is the smallest letter in the Gr. alphabet and here it signifies yod, the smallest letter of the Heb. alphabet. The expression “not an iota, not a dot” signifies the tiniest details. It is used by Jesus to emphasize the enduring and unchangeable nature of the law of God (i.e., The Torah, the OT). Similar statements are found in Jewish tradition asserting that a yod could not be removed from the law, that the world would be destroyed if one mark which distinguishes similar letters were removed.
A. Edersheim, Jesus the Messiah (1947), I, 537f.; W. Arndt and F. Gingrich, A Greek-English Lexicon of the NT (1957), 429.