“unyielding.” The Hebrew word is kabad (#03513 כָּבַד), meaning unyielding, hard, insensitive, stubborn, unresponsive, dull. See commentary on Exodus 4:21 and 10:1.
“his servants.” In this context, “servants” refers to the officials who serve the Pharaoh, not his household servants. The HSCV nuances this to “officials,” which is not literal but is very clear.
[For more on the word “servants” being used for people of high position in the kingdom, see commentary on 2 Samuel 11:1.]