When Thetis comes to Zeus in the 24th book of the Iliad, the father of gods and men seems to empathize with her.
“You have come to Olympus, divine Thetis, though in grief, carrying unforgettable sorrow in your heart. I know this myself.”
ἤλυθες Οὔλυμπον δὲ θεὰ Θέτι κηδομένη περ,
πένθος ἄλαστον ἔχουσα μετὰ φρεσίν· οἶδα καὶ αὐτός· Il. 24.104-105
Eustathius found the use of the adjective alaston here particularly interesting.
Eustathius, Comm ad Il. 4.875
“To have grief unceasing (which is from epalastein, to be troubled) should be translated as “grieving” [as in a funeral], since feeling the affliction of mourning is in these words, grieving. This grief is in fact for Achilles who is still living; Thetis has drawn the dark veil across her face as is appropriate for those mourning over corpses.”
(v. 105) Τὸ δὲ πένθος ἔχειν ἄλαστον, ἐξ οὗ τὸ ἐπαλαστεῖν, ἐφερμηνευτικόν ἐστι τοῦ κήδεσθαι, ἵνα ᾗ ἐν τοῖς ῥηθεῖσι κήδεσθαι τὸ πενθικῶς βλάπτεσθαι. (v. 93 s.) ῞Οτι διὰ πένθος τὸ ἐπὶ ᾿Αχιλλεῖ, καὶ ταῦτα ζῶντι ἔτι, κάλυμμα ἡ Θέτις ἕλε κυάνεον, ὡς εἰκὸς τοὺς ἐπὶ νεκροῖς παθαινομένους. «τοῦ δ’ οὔ τι μελάντερον ἔπλετο ἔσθος».
In an earlier passage, Eustathius acknowledges that this adjective may have a broader semantic field:
Eustath. Comm ad Il II 4.613
“Look how wise and fitting it is that Achilles calls Hecktor alastos, because he is grieving unceasingly. For as he hurries to this task he does not take pleasure in the insult. Not only does the word alastôr (“avenger; sinner”) derive from alastos, but it also provides alastein (ἀλαστεῖν; “to be angry”) and the compound epalastein (ἐπαλαστεῖν; “to be troubled”) which appears in the Odyssey.
῞Ορα δὲ, ὡς ἠρκέσθη νῦν σωφρόνως ὁ ᾿Αχιλλεὺς ἄλαστον εἰπεῖν τὸν ῞Εκτορα, ὡς ἀλάθητα λυπήσαντα. σπεύδων γὰρ εἰς ἔργον ὕβρεσιν οὐκ ἐνευκαιρεῖ. ἐκ δὲ τοῦ ἄλαστος οὐ μόνον ὁ ἀλάστωρ παράγεται, ἀλλὰ καὶ τὸ ἀλαστεῖν καὶ συνθέτως δὲ ἐπαλαστεῖν, ὅπερ ἐν ᾿Οδυσσείᾳ κεῖται.
The etymology of the adjective might look like this:
ἄλαστος: alastos, likely from the root *lath-, “escape memory”; cf. Gk. λανθάνω (“escape notice”); ἀληθής (“true”); λήθη (“forgetfulness”). It is also realted to ἀλάστωρ (alástôr, “avenger”) as “one who does not forget”. Ancient folk etymologies also relate it falsely to alaomai (“to wander”) and some modern scholars have suggested a visual root (e.g. laô). See Chaintraine s.v. ἀλάστωρ..
Post-epic, the adjective more often means “cursed” (cf. Soph. Oed. 1483: ἄλαστον ἄνδρ’) and the root is productive in verbs like alastein (ἀλαστεῖν; “to be angry”) and the compound epalastein (ἐπαλαστεῖν; “to be troubled”).