Abbott-Smith Manual Greek Lexicon
ἀτακτέω (G812)
1 * 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.).†
G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922) — public domain