κρουστέον
From LSJ
Περὶ τοῦ ἐπέκεινα τοῦ νοῦ κατὰ μὲν νόησιν πολλὰ λέγεται, θεωρεῖται δὲ ἀνοησίᾳ κρείττονι νοήσεως → On the subject of that which is beyond intellect, many statements are made on the basis of intellection, but it may be immediately cognised only by means of a non-intellection superior to intellection
English (LSJ)
one must knock at, τηνδεδί (sc. θύραν) Ar.Ec.989.
French (Bailly abrégé)
adj. verb. de κρούω.
Greek (Liddell-Scott)
κρουστέον: ῥημ. ἐπίθ., δεῖ κρούειν, τὴν θύραν Ἀριστοφ. Ἐκκλ. 989.
Russian (Dvoretsky)
κρουστέον: adj. verb. к κρούω.
Dutch (Woordenboekgrieks.nl)
κρουστέον, adj. verb. van κρούω, er moet gebeukt worden.