Lit., one another - yourselves. For a similar variation of the pronoun see Eph 4:32; 1Pe 4:8-10. The latter pronoun emphasizes the fact that they are all members of Christ's body - everyone members one of another - so that, in forgiving each other they forgive themselves.
Only here in the New Testament. Cause of blame. Rev., complaint. The A.V. uses quarrel in its earlier sense of cause of complaint. So Shakespeare:
“The lady Beatrice hath a quarrel to you.”
“Much Ado,” ii., 1.
“Against whom comest thou, and what's thy quarrel?”
“Richard II.,” i., 3, 33.
Holinshed: “He thought he had a good quarrel to attack him.” It was used of a plaintiff's action at law, like the Latin querela.