A lean and hungry look
Let me have men about me that are fat,
Sleek-headed men and such as sleep a-nights.
Yond Cassius has a lean and hungry look,
He thinks too much; such men are dangerous.
Cassius appears a little underfed these days. His "lean and hungry look" unsettles Julius Caesar, who prefers the company of fat, contented men—who wouldn't bite the hand that feeds them. Cassius looks like he's been up late nursing his envy, a situation that bodes ill for the dictator. Caesar's intuition is accurate: Cassius will spearhead the plot to assassinate him [see MASTERS OF THEIR FATES].
Marcus Antonius tries to soothe Caesar: "Fear him not, Caesar, he's not dangerous,/ He is a noble Roman, and well given." Ironically, the superstitious Caesar, who sees through Cassius's noble exterior, will die, while the deluded Antony will survive to avenge him, and later to take power.