According to SnΓ¦dal (2015; see references), from Ancient GreekΟ‘(Ε‘, βsampiβ), which shares the same numerical value and had a very similar shape in antiquity (see its Wikipedia article for more information on its past forms).
Snædal, 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 98.