Lexical Definition

‎ἀχάριστος‎ acháristos (ach-ar'-is-tos) ungrateful ^G:A
from ‎Α‎ (as a negative particle) and a presumed derivative of ‎χαρίζομαι‎

unthankful.

• thankless, i.e. ungrateful

Abbott-Smith

** ἀχάριστος, -ον (< χαρίζομαι), [in LXX: Wi 16:29, Si 29:17, 25, IV Mac 9:10*;]
(a) ungracious, unpleasing;
(b) ungrateful, thankless: Lk 6:35, II Ti 3:2.†

Abbott-Smith References

Liddell-Scott-Jones

ἀχάριστος-ος, ον,

(χᾰρίζομαι) ungracious, unpleasant, οὐκ ἀχάριστα μεθ᾽ ἡμῖν ταῦτ᾽ ἀγορεύεις[Refs 8th c.BC+]; without grace or charm, οὐκ ἀχάριστα λέγειν[Refs 5th c.BC+]; -ότερον ἐπιμέλημα a more thankless business, [Refs 5th c.BC+]

__II of persons, ungracious, unfavourable, [Refs 6th c.BC+]
__II.2 ungrateful, thankless, [Refs 5th c.BC+]; σπείρων εἰς ἀχάριστα sowingin thankless soil, [Refs 1st c.BC+]. adverb -τως, ἀποπέμψασθαι εὐεργέτας[Refs 5th c.BC+]
__II.3 passive, unrequited, ἀ. εἶναι τὰ ἀνηλωμένα[Refs 5th c.BC+]adverb οὐκ ἂν ἀχαρίστως μοι ἔχοι πρός τινος thanks would not be refused me by.., [Refs 5th c.BC+]
__II.4 adverb -τως with a bad grace, with an ill will, ἀ. ἕπεσθαι follow sulkily, [Refs 5th c.BC+]
__II.5 ἀχάριστον, τό, antidote, [Refs 2nd c.AD+]
__II.5.b name of an eyesalve, [Refs 1st c.AD+]