“I am a great eater of beef, and I believe that does harm to my understanding.”

Boswell records Johnson making the remark about his own diet, and the joke is that the great critic, who had opinions on everything, suspected his appetite of dulling him. The self-deprecation is also a convenient way of explaining a lifelong preference for plain food, and the line is the standard quotation whenever a scholar complains that his mind has slowed down.