We don’t hear the expression “jot or tittle” very commonly nowadays, though it crops up on occasion and we find it in the Bible, as in Christ’s Sermon on the Mount.
The expression as a whole means a minute particle. And that’s just what it always meant.
Jot is a corrupted form of the Greek iota; the Greek letter i, in other words. The corrupted spelling, with initial j instead of i, goes back to the days before the eighteenth century when i and j were used almost interchangeably.
The iota is the smallest letter in the Greek alphabet, scarcely requiring more than a wiggle of the stylus to produce it. And tittle means the dot over the i, a mere point.
Anciently it meant any point by which pronunciation was indicated, just as we use tilde, a corrupted form of tittle, for the wavy line over an n in some Spanish words, like canon.
So, by the expression jot or tittle, the ancients really meant something so small as to be scarcely noticed, not only the tiny letter;, but the even tinier dot or point over the i.