A tittle is originally a small mark either on or in a letter, i.e. it may be part of the letter or it may be a diacritic (accent) near it.
Webster 1913 is out on a limb with the suggested derivation from tit 'small thing, spec. small thing on the front of a lady's body'. More reliable dictionaries straightforwardly say tittle is in origin the same word as title.
Title comes from Latin titulus, originally an inscription or sign or label on or above any object (not just a book). So, on a letter, a title or tittle was a distinguishing mark on or above it. In English it was originally used for what was called the apex in Latin, the oblique stroke sometimes used above vowels to indicate long vowels. (Modern Latin pedagogic texts use a macron to show long vowels, but in ancient Latin they used - occasionally - the acute accent, e.g. Á É.)
So a tittle can be a diacritic, or the dot over i and j, or a serif, or a swash indicating an abbreviation, and so on. From this sense came the figurative one of a 'tittle' being something very small. (Unless Webster is right, and frankly my dear, in ten rounds with Oxford...)
Specifically, the tittle is the little sticky-outy bit on the edge of many Hebrew letters. Many pairs of letters in the (printed) Hebrew alphabet are distinguished only by the fact that one has this extra serif or hook, e.g. resh vs daleth, or kaph vs beth. The small letter yod (Y) consists of little more than a tittle by itself.
The Greek iota (I) is related to the Hebrew yod (and to Arabic ya and Roman I); a former English name for iota was jot. (J and I are originally the same letter.) It, like yod, is the smallest letter in its alphabet. Thus in Greek an iota or jot could be a very small thing.*
Thus the use of jot or tittle to mean two very small things which could make a difference in meaning to a word, so if you want to be precise you have to get every jot and tittle correct. This pairing came into English in the biblical translation of Matthew 5, verse 18:
Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
* In an earlier version of this write-up I included the following information. It is in fact not relevant, because the iota subscript was not yet in use at the time that St Matthew was writing his Greek gospel. I retain it as a note because I was repeating what I had seen asserted, and I now want to clarify that this is irrelevant:
Now in classical Greek there were long diphthongs âi êi ôi written with iota. In post-classical Greek the i sound disappeared but continued to be written; later on this was indicated by writing it as an iota subscript, a small iota underneath the other letter.
Webster also omits the dialectal word 'tittle', probably
onomatopoeic, meaning '
whisper;
tattle', and which gets combined in the more familiar
tittle-tattle.