James 5:9 - Matthew Poole's English Annotations on the Holy Bible

Grudge not; Greek: Groan not; the sense may be, either: Envy not one another, (or, as we translate it: Grudge not), it being the nature of envy to groan at other men's good; or, Groan not by way of accusation or complaint to God against others, desiring him to avenge your quarrels, as if you were too good to suffer injuries, or God were unjnst or forgetful of righting you. One against another; brother against brother, Christian against Christian: they were injured not only by rich worldlings and open oppressors, but by their fellow professors, and gave one another mutual cause of sighing and groaning. Lest ye be condemned; lest God punish you all; there being none of you but have given others cause of grief and complaint, as well as others have given you, Matthew 7:1. Behold, the Judge standeth before the door; the Lord Jesus Christ, the Judge of you all, is at hand, \Philippians 4:5\ in a readiness either to bring those evils upon you which you wish may fall upon others, or to give you your reward, if through patient continuance in well doing you seek for it, Romans 2:7. The like phrase we have, Matthew 24:33 Mark 13:29; or it may allude to Genesis 4:7.