dementio
From LSJ
ἐπ' αὐτὸν ἥκεις τὸν βατῆρα τῆς θύρας → you've come to the crux of the matter, come to the point, hit the nail on the head, you've come to the very threshold of the door, you are come to the very threshold of the door, you've arrived at the truth of the matter
Latin > English (Lewis & Short)
dēmentĭo: īre, 4, v. n. id.,
I to be out of one's senses, to be mad, to rave (anteand post-class.): dementit deliraque fatur, * Lucr. 3, 464: sese mea magia in amorem inductam dementire, App. Mag. p. 324, 9: aliquis instinctu daemonis percitus dementit, effertur, insanit, Lact. 4, 27 med.>
