Jamieson, Fausset and Brown on Isaiah 29:21

21. Rather, "Who make a man guilty in his cause" [Gesenius], that is, unjustly condemn him. "A man" is in the Hebrew a poor man, upon whom such unjust condemnations might be practiced with more impunity than on the rich; compare Is 29:19, "the meek ... the poor."

him that reproveth--rather, "pleadeth"; one who has a suit at issue.

gate--the place of concourse in a city, where courts of justice were held (Ru 4:11; Pr 31:23; Am 5:10, 12).

just--one who has a just cause; or, Jesus Christ, "the Just One" [Horsley].

for a thing of naught--rather, "through falsehood," "by a decision that is null in justice" [Barnes]. Compare as to Christ, Pr 28:21; Mt 26:15; Ac 3:13, 14; 8:33.