Charles Ellicott Commentary Psalms 137:5

"If I forget thee, O Jerusalem, Let my right hand forget [her skill]." — Psalms 137:5 (ASV)

Her cunning — i.e., the skill of playing on the harp. If at such a moment the poet can so far forget the miserable bondage of Jerusalem as to strike the strings in joy, may his hand forever lose the skill to touch them.