Lexical Definition

‎ἄτακτος‎ átaktos (at'-ak-tos) disorderly ^G:A
from ‎Α‎ (as a negative particle) and a derivative of ‎τάσσω‎

unruly.

• unarranged, i.e. (by implication) insubordinate (religiously)

Abbott-Smith

** ἄ-τακτος, -ον (< τάσσω), [in LXX: III Mac 1:19*;]
out of order, out of place (Lat. inordinatus), freq. of soldiers not keeping the ranks, or an army in disarray (cf. III Mac 1:19); hence, metaph., irregular, disorderly (v. previous word): I Th 5:14.†

Abbott-Smith References

Liddell-Scott-Jones

ἄτακτος-ος, ον,

not in battle-order, of troops, [Refs 5th c.BC+]

__2 not at one's post, [Refs 4th c.BC+]
__II undisciplined, disorderly, θόρυβος[Refs 5th c.BC+]; irregular, πυρετός[Refs 5th c.BC+]casual, [Refs]; of sensual excess, irregular, inordinate, ἡδοναί, Ἀφροδίτη, [Refs 5th c.BC+]; in Music, without rhythm, μελῳδίαι[Refs 3rd c.AD+]; in Medicine texts, irregular, σφυγμός[Refs 2nd c.AD+]
__II.2 uncivilized, lawless, βίος[Refs 5th c.BC+]
__II.3 in Mathematics texts, ἄτακτοςα προβλήματα indeterminate, not admitting of a definite solution, [Refs 5th c.AD+]
__B adverb -τως in an irregular, disorderly manner, of troops, ἀ. καὶ οὐδενὶ κόσμῳ προσπίπτοντες[Refs 5th c.BC+]
__B.2 irregularly, of fevers, [Refs 5th c.BC+]
__B.3 comparative ἀτακτοτέρως somewhat negligently, [Refs 4th c.BC+]