taetrum
From LSJ
θεωρεῖται δὲ ἀνοησίᾳ κρείττονι νοήσεως → it is grasped only by means of an ignorance superior to intellection, it may be immediately cognised only by means of a non-intellection superior to intellection
tætrum, adv., d’une manière affreuse : Prud. Cath. 4, 22.