Psalms 137:4 - Matthew Poole's English Annotations on the Holy Bible

The Lord's song; those songs which were appointed by God, and to be sung only to his honour and in his service. In a strange land; when we are banished from our own temple and land, and amongst those who are strangers and enemies to God and to his worship. So we should prostitute and profane God's ordinances. And this answer they either expressed to their enemies, or kept in their own breasts when they refused to comply with their desire.