πλακούντιον
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)
τό, Dim. of πλακοῦς, Arr.Epict.2.16.25, Aët.9.30, Sch.Porph.Abst.2.16 :— written πλᾰκουντ-όντιον, PLond.3.964.20 (iii A.D.).