An Etymological Dictionary of the German Language/Stier
Stier, m., ‘bull,’ from the equiv. MidHG. stier, OHG. stior; a common Teut. term; comp. Goth. stiur, AS. steór, E. steer, Du. stier. The remarkable variant OIc. þjórr, Dan. tyr, Swed. tjur, points to pre-Teut. teuro- and steuro-; to this OSlov. turŭ, ‘bull,’ Zend staora, ‘draught cattle,’ and the Sans. adj. sthûra, ‘great, mighty’ (OIc. stórr, OHG. stûri), are perhaps allied. Gr. ταῦρος (whence Lat. taurus) is based, as is indicated by OIr. tarb, on a primit. form tarwos.