What Lear is saying here can be interpreted in different ways. According to the OED definition 4a, “mad” can also be defined as a person who is “insane, crazy, or mentally deranged.” On one hand, his daughters are making him incredibly angry, but he also may be hinting that they are actually driving him mentally insane.

“mad, adj.” OED Online. Oxford University Press, March 2017. Web. 17 April 2017.