Okay. I realise now that this could have been better situated in grammar / (implied) syntax / voice rather (i.e. asking the question, "What does φοβεῖσθαι mean here in this construction?" ) than in "Word Meanings" (i.e. asking the question, "What does φοβεῖσθαι mean?" per se
), but anyway let's continue...
I would assume that Luke is cognizant of if not even drawing upon the Marcan text:
14:1-2 ... καὶ ἐζήτουν οἱ ἀρχιερεῖς καὶ οἱ γραμματεῖς πῶς αὐτὸν ἐν δόλῳ κρατήσαντες ἀποκτείνωσιν· 2 ἔλεγον γάρ· μὴ ἐν τῇ ἑορτῇ, μήποτε ἔσται θόρυβος τοῦ λαοῦ.
and that they feared popular support of Jesus as Messiah would bring down a new repressive Roman reaction that could upset their own status and authority in Judea.
Okay. My concern with that reading (which I also prefer) is that now that deponent is defunct, that is a long road to a middle "voice". The Jewish leaders are afraid that the people might make Jesus king by force at the festival and then the Romans would put down the uprising by force and remove some or all of the tollerance that they had granted the Jews up until that point, and then
the Jewish leaders would no longer have their power. There are a lot of notes being played with a single draw of the bow. Could you explain the use of this accusative with the middle.
A follow-up question;
If the verb were to be supplied after the φοβεῖσθαι, would it be used directly, or would a μὴ "lest" preceed it? And what aspect / mood / tense would that verb be? (Perhaps we could work from some or other of the verbs in John 6:15 (ἔρχεσθαι καὶ ἁρπάζειν αὐτὸν ἵνα ποιήσωσιν αὐτὸν βασιλέα) to syntactically expand this statement out into the sense we conjecture - understand what might have been said but wasn't - and then better understand what was said.
Οὐκ οἶδα τί λέγεις. ¯\_(ツ)_/¯