What does King Lear mean when he says "Let me wipe it first; it smells of mortality"?

In Act IV, Scene 6, there is a pathetic and funny meeting between two old men, Lear and Gloucester, who have lost everything because they trusted their children. Gloucester says:

O, let me kiss that hand.

This is when Lear says:

Let me wipe it first: it smells of mortality.

Lear has been living out in the open country for a long time. He is ragged and dirty, but still has some courtly manners. He...

In Act IV, Scene 6, there is a pathetic and funny meeting between two old men, Lear and Gloucester, who have lost everything because they trusted their children. Gloucester says:

O, let me kiss that hand.

This is when Lear says:

Let me wipe it first: it smells of mortality.

Lear has been living out in the open country for a long time. He is ragged and dirty, but still has some courtly manners. He is still concerned about how he looks and how he smells. Shakespeare intended this line, and the action that accompanies it, to get a laugh from his audience. Lear smells his own hand first and then wipes it on his rags. Everyone knows what he means when he says it smells of mortality" Lear smells his own excrement on his hand. He has been relieving himself out of doors as best he can, but hasn't been able to wash his hands afterwards. The implication is that he hates humanity by now and that the smell on his hand reminds him of his opinion of humanity, or mortality. This is why the audience would laugh. There is a lot of action to go with the line. Lear smells his hand first, then wipes it, then says it smells of mortality. The audience expects him to say it smells of something else first, but he pauses and considers, and then says "mortality" instead of a different (four-letter) word.

William Faulkner, who had a cynical opinion of humanity, was quoted as saying something similar about humankind: 

When [Malcolm] Cowley, for example, wrote asking if it would be fair to call his work a “myth or legend of the South,” Faulkner testily replied that the South “is not very important to me,” adding, in a gratuitous discharge of bile, that in his opinion human life is “the same frantic steeplechase toward nothing everywhere and man stinks the same stink no matter where in time.”
Frederick Crews, “Faulkner Methodized,” in The Critics Bear It Away: American Fiction and the Academy