Quack (2017). “How the Coptic Script Came About”. Greek Influence on Egyptian-Coptic: Contact-Induced Change in an Ancient African Language. Widmaier Verlag. p. 75. https://www.academia.edu/42127007. "It has normally been claimed that it derives from the form of the infinitive ti in Demotic, but the actual forms do not fit well; and furthermore it is a point of some concern that this sign never turns up in any ‘Old Coptic’ text (where we always have ⲧⲓ for this sound sequence). For this reason the proposal by Kasser that it is actually a ligature of t and i seems to me quite convincing."
