Where does the Ge'ez letter psa come from?

Why it is open. Psa writes a plain p, a sound Ge'ez did not inherit, because the Semitic p had become f in this branch. The letter therefore has to be an addition to an order that was already fixed, and two explanations circulate: that it is the Greek pi, borrowed with the Greek names and vocabulary that made a p letter necessary, and that it is a modification of an existing fidel sign. Both are readings of the letterform and of its position at the end of the order. Neither rests on a document.

What would resolve it. Dated Aksumite and post-Aksumite inscriptions and manuscripts showing the earliest forms of psa and payt, read against the Ancient South Arabian signary and against Greek letterforms in use in Aksum, would show whether the sign entered the script from outside or was built inside it.

Field. Ethiopian epigraphy and Semitic palaeography