There I was, working through Kirsopp Lake's Greek and translation of the Shepherd of Hermas, and I came across Mandates V.ii.4:
4 ἡ δὲ ὀξυχολία
πρῶτον μὲν μωρά ἐστιν,
ἐλαφρά τε καὶ ἄφρων.
εἶτα ἐκ τῆς ἀφροσύνης γίνεται πικρία,
ἐκ δὲ τῆς πικρίας θυμός,
ἐκ δὲ τοῦ θυμοῦ ὀργή,
ἐκ δὲ τῆς ὀργῆς μῆνις·
εἶτα ἡ μῆνις αὕτη·
ἐκ τοσούτων κακῶν συνισταμένη·
γίνεται ἁμαρτία μεγάλη καὶ ἀνίατος.
(Hermas 34.4 || Mandates V.ii.4, Lake's Greek)
4 But ill temper
is first foolish,
frivolous, and silly;
then from silliness comes bitterness,
from bitterness wrath,
from wrath rage,
and from rage fury;
then fury,
being compounded of such great evils,
becomes great and inexpiable sin.
(Hermas 34.4 || Mandates V.ii.4, Lake's English)
Upon seeing ἀνίατος translated as "inexpiable", I thought to myself, "now that's a word to remember." The Greek word is from ἰάομαι (alpha privative) and glossed generally as "incurable" in BDAG; BDAG offers a specific translation of this passage as "unforgivable". I don't like "unforgivable" here because the author of Hermas uses terms like forgiveness a whole lot. If he specifically meant "unforgivable", then he had an ample lexicon to produce that. But he didn't. He wrote ἀνίατος.
I like Lake's take on it. Holmes translates the same as BDAG's gloss, "incurable", and that matches up with LSJ. But I still like "inexpiable". To think about something as "unable to be expiated" brings the seriousness of it into play, much more so than works like "unforgivable" (which is accurate of the result). A translation like "incurable" makes it sound like more of a malady. I think "inexpiable" threads the needle between those two, and is simply a cool word to boot.
Score one for Kirsopp Lake. FWIW, I generally find Holmes' translation better and more readable, but Lake has enough gems that it is well worth considering. And in most books, Lake tends to be less idiomatic and more "literal" (whatever that means), so it is easier to use as a check when working through the Greek.