If I forget thee, O Jerusalem, let my right hand forget her cunning.
If I forget thee, O Jerusalem, let my right hand forget (her cunning) - let my right hand no longer perform its function of playing the harp (cf. Psalms 76:5, close; Job 31:22). A fit retribution-that my right hand should be deprived altogether of power to play, if, forgetting thee, Jerusalem, it should play thy joyful songs while I am in exile from thee.