jot (n.)

"very small amount," 1630s, figurative use of iota, ninth and smallest letter in the Greek alphabet (corresponding to Latin -i-). Its use in this sense is after Matthew v.18 (see jot (n.), which is the earlier form of the name in English), but iota in classical Greek also was proverbially used of anything very small. The letter name is from Semitic (compare Phoenician and Hebrew yodh).

"small stroke or mark made by a pen-point in writing," late 14c., titil (Wycliffe, in Matthew v.18); it is title (n.) with a specialized sense (which developed in Late Latin and Romanic) and pronunciation. In Wycliffe it translates Latin apex in the Late Latin sense of "accent mark over a vowel," which itself translates Greek keraia (literally "a little horn"), used by the Greek grammarians of the accents and diacritical points. In this case the Greek word is a Biblical translation of Hebrew qots, literally "thorn, prick," used of the little lines and projections by which the Hebrew letters of similar form differ from one another.

Wycliffe's word is borrowed from a specialized sense of Latin titulus, which was used in Medieval Latin (and in Middle English and Old French) to indicate "a stroke over an abridged word to indicate letters missing" (compare Provençal titule "the dot over -i-", and tilde, which represents in English the Spanish form of the same word; also see iota).

As apex was used by the Latin grammarians for the accent or mark over a long vowel, titulus and apex became to some extent synonymous; hence Wyclif's use of titil, titel to render L. apex [OED]

As "smallest or very small part of something" by late 14c. (compare jot (n.)). The phrase to a tittle "with great exactness" is from c. 1600.