"A man may fish with the worm that hath eat of a king, and eat of the fish that hath fed of that worm." What is Hamlet saying in this line?...

In Act IV Scene 3 of Hamlet, the
prince of Denmark controls the scene even though he gives the pretense of being mad. 
With Claudius he mitigates the seriousness of the situation and the importance of
Claudius's position by engaging in word games with him.  For instance, Hamlet bids
good-bye to Claudius by contradicting the king and insulting him after he calls himself
Hamlet's father; instead, Hamlet calls him "mother," explaining that if Claudius is
married to his mother, and man and wife are "one flesh," then Claudius is his
mother.

Likewise, Hamlet bandies with words in such a way
as to deprecate the court of Denmark.  When Claudius asks Hamlet the location of
Polonius, Hamlet replies that Polonius is "at supper," but it is he who is supper, the
supper of worms.  He then speaks of the food chain of life, certainly a chain that is
humbling if one thinks of it.  Hamlet says that a man can fish with a worm that has,
like the worms eating Polonius, fed in the earth filled with the decay of a king.  Then,
the man can eat the fish that has been caught with that worm, thus making him indirectly
the eater of the king, also.  Of course, in his pretended madness, Hamlet again hints at
Claudius's heinous act of regicide.  For, by becoming the wife of Hamlet's mother,
Claudius, the murderer of King Hamlet, is the man who "eats the fish that hath fed of
that worm."