According to legend, Cain killed his brother Abel with the jawbone of an ass. Here, in mock revenge, Cain’s skull is being jowled (thrown) by an ass (the gravedigger). Shakespeare makes a pun with “jowl’, which also means jawbone. He’s also saying that Cain could be considered the first politician, so when God asked him where his brother was, he avoided giving a direct answer, he “circumvented” God.