Jamieson, Fausset and Brown on 1 Thessalonians 4:6

6. go beyond--transgress the bounds of rectitude in respect to his "brother."

defraud--"overreach" [Alford]; "take advantage of" [Edmunds].

in any matter--rather as Greek, "in the matter"; a decorous expression for the matter now in question; the conjugal honor of his neighbor as a husband, 1Th 4:4; 1Th 4:7 also confirms this view; the word "brother" enhances the enormity of the crime. It is your brother whom you wrong (compare Pr 6:27-33).

the Lord--the coming Judge (2Th 1:7, 8).

avenger--the Righter.

of all such--Greek, "concerning all these things;" in all such cases of wrongs against a neighbor's conjugal honor.

testified--Greek, "constantly testified [Alford].