16 “Render,” he says, “to our neighbours seven times so much into their bosoms”. Not any evil things he is wishing, but things just he is foretelling and prophesying as to come. But in the number seven, that is, in sevenfold retribution, he would have the completeness of the punishment to be perceived, for with this number fullness is wont to be signified. Whence also there is this saying for the good, “He shall receive in this world seven times as much:” which has been put for all.
“As if having nothing, and possessing all things.” Of neighbours he is speaking, because among them dwells the Church even unto the day of severing: for not now is made the corporal separation. “Into their bosoms,” he says, as being now in secret, so that the vengeance which is now being executed in secret in this life, hereafter may be known among the nations before our eyes. For when a man is given over to a reprobate mind, in his inward bosom he is receiving what he deserves of future punishments.
“Their reproach wherewith they have reproached You, O Lord.” This do You render to them sevenfold into their bosoms, that is, in return for this reproach, most fully do You rebuke them in their secret places. For in this they have reproached Your Name, thinking to efface You from the earth in Your servants.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)