πρωκτὸς λουτροῦ περιγίνεται· ὅταν τις μὴ δύνηται ἀπονίψασθαι, ἀλλ’ ἡ κοιλία αὐτῷ ἐπιφέρηται· * * τῶν ἀνωφελῶν.
Τhe anus overwhelms the bath: whenever someone is not able to wash, but the feces accumulates on him. [ * * ] of worthless/harmful things/people.
The editors think there's text missing where the asterisks are.
The verb περιγίγνεται, which I am used to seeing in battle accounts, makes this proverb especially vivid for me.
There is an extensive footnote on this one which people with Latin and Greek both may find interesting. Several of the scholia seem to point to more precise, if somewhat contradictory, senses of the proverb:
παροιμιακὸν τοῦτο ἐπὶ τῶν ἐπὶ κακῷ τῷ ἑαυτῶν νικώντων· ἢ ἐπὶ τῶν ἀεὶ μολυνομένων καὶ βιαζομένων καθαίρεσθαι.
This proverb is about those conquering their own problems; or about those who constantly defile themselves and are forced to clean up.
Καλλίστρατος δέ φησι· παροιμία, πρωκτὸς λουτροῦ περιγνίνῃ, ἑπὶ τῶν βιαζομένων εἰς κακὸν ἑαυτούς· ὡς εἴ τις βιάζοιτο μὴ ἀποπλύνεσθαι.
Callistratus says, "a proverb, 'the anus overcomes the bath,' used about those compelled to do themselves ill, as if someone were forced to not wash."
There is a huge ambiguity in the first quote concerning the infinitive καθαίρεσθαι. The base sense is "cleanse, wash off" but it also has a specific medical sense, "purge, evacuate," which might include pharmacological aid (see the LSJ entry on καθαίρω).
Scholiastic Greek, even with the help of Dickey's book, is still often a puzzle, so I'll happily accept other suggestions for translation.