Charles Ellicott Commentary Exodus 5:9

"Let heavier work be laid upon the men, that they may labor therein; and let them not regard lying words." — Exodus 5:9 (ASV)

Let them not regard vain words. —Or, false words. The reference is to the promises of deliverance with which Moses and Aaron had raised the people’s hopes (Exodus 4:30). Pharaoh supposed these to be “vain words,” as Sennacherib did those spoken by Hezekiah (2 Kings 18:20).