Lexical Definition

‎ἀθέτησις‎ athétēsis (ath-et'-ay-sis) nullification ^G:N-F
from ‎ἀθετέω‎

disannulling, put away.

• cancellation (literally or figuratively)

Abbott-Smith

† ἀθέτησις, -εως, ἡ (< ἀθετέω), [in LXX, usually of unfaithful, rebellious action: I Ki 24:12* (פֶּשַׁע), Je 12:1 (בֶּגֶד), Da TH 9:7 (מַעַל), II Mac 14:28*;]
a disannulling, setting aside: He 7:18 9:26 (for similar usage in π., v. Deiss., BS, 228 f.; MM, VGT, s.v.).†

Abbott-Smith References

Liddell-Scott-Jones

ἀθέτησις-ησις, ἡ,

a setting aside, abolition, ἁμαρτίας[NT+2nd c.AD+]; 'annulling' of a deed, [Refs 1st c.AD+]

__II rejection (of a spurious passage), [Refs 2nd c.AD+]; generally, rejection, opposed to ἐποχή, [Refs 1st c.BC+]
__III breach of faith, [Refs 2nd c.AD+]

_{LSJ Scripture Refs: Heb.9.26}