Lexical Definition

‎ἀτακτέω‎ ataktéō (at-ak-teh'-o) be idle ^G:V
from ‎ἄτακτος‎

behave self disorderly.

• to be (i.e. act) irregular

Abbott-Smith

* ἀτακτέω, -ῶ (< ἄτακτος),
primarily, of soldiers marching, to be out of order, to quit the ranks; hence, metaph., to be remiss, fail in the performance of duty (in π., of truancy on the part of an apprentice): II Th 3:7 (on ἀ. and its cognates, v. M, Th., 152 ff.; MM, s.vv.).†

Abbott-Smith References

Liddell-Scott-Jones

ἀτακτέω-έω,

of a soldier, to be undisciplined, opposed to εὐτακτέω, [ἀτακτέωοῦντας τῶν τριηράρχων “IG” 2.809b13">Refs 5th c.BC+]:—passive, πολλὰ γὰρ ἠτάκτητο αὐτοῖς[Refs 1st c.AD+]: generally, neglect one's duty, fail to discharge obligation, [NT+5th c.BC+]

__2 generally, lead a disorderly life, [Refs 5th c.BC+] to desert it, [Refs 1st c.AD+]
__3 raise a riot or rebellion, [Refs 4th c.AD+]

_{LSJ Scripture Refs: 2Thess.3.7}