Uncertain; has been related to Ancient Greekฮ(Th, โthetaโ), ฮจ(Ps, โpsiโ) and ฮฆ(Ph, โphiโ). Magnus Snรฆdal (2015; see references) considers the derivation from phi most likely.
Snรฆdal, Magnus, 'Gothic Contact with Latin: Gotica Parisina and Wulfila's Alphabet', in: Askedal, J.O. and H.F. Nielsen ed., Early Germanic Languages in Contact (Amsterdam 2015) pp. 91-108, especially 99-100.