βουκράνιον
From LSJ
αὐτὸν κέκρουκας τὸν βατῆρα τοῦ λόγου → you have struck the very threshold of the argument, you have struck the most important and chiefmost point
English (LSJ)
[ᾱ], τό,
A ox-head, EM207.55. II = ἄμπελος μέλαινα, Ps.-Dsc.4.183. 2 = ἀντίρρινον, ib.130, Gal.19.82. III machine for reducing dislocations, Orib.49.4.74.