It's biblical, from the New Testament. Matthew 5.18:

ἀμὴν γὰρ λέγω ὑμῖν, ἕως ἂν παρέλθῃ ὁ οὐρανὸς καὶ ἡ γῆ, ἰῶτα ἓν ἢ μία κεραία οὐ μὴ παρέλθῃ ἀπὸ τοῦ νόμου ἕως ἂν πάντα γένηται.

For amen, I say to you, until such time as the sky and the earth pass away, not one iota nor one keraia is to pass away from the law until such time as everything takes place.

As a point of interest, the King James translation rendered iōta and keraia as "jot and tittle", where "jot" is an anglicisation of iōta and "tittle" refers to the dot over a lower case i or j.

The actual intended meaning of the words is to some extent up for grabs. Iōta may be simply an example of a letter, or it may have been chosen to refer to the smallest of the Greek letters, in terms of how much space it occupied in a manuscript (in Greek as in the Roman alphabet, I takes up less space on a line than any other letter).

Since keraia means "anything horn-shaped", it has been variously interpreted as "an accent", "a pen stroke", or "a comma". "Pen stroke" is the most accurate, as indicated e.g. by this parallel in the pseudo-Galenic work On weights and measures:

Τὸ δὲ Κ στοιχεῖον, εἰ μὲν ἔχει ε ἐπικείμενον αὑτῷ καὶ λοξὴν εὐθεῖαν γραμμὴν τέμνουσαν τὴν κάτω κεραίαν αὐτοῦ, κεράτιον δηλοῖ...

The letter K, if it has an epsilon written above it and a straight diagonal line crossing its lower keraia, represents a carat.

So here the "lower keraia" of the K is its lower diagonal stroke.

According to a glossary in Eleanor Dickey's Ancient Greek Scholarship, keraia could also refer to the tip of a letter (which is why the Vulgate translates it with the Latin word apex), but she doesn't cite the sources for that; I'd say "stroke" makes more sense anyway.