act 2 scene 2

Hamlet is doing double-speak the whole conversation with Polonios (Act 2 Scene 2), alternately trying to make Polonios think that he is mad, or insulting the older man.

HAMLET: Excellent well. You are a fishmonger.

Here Hamlet is insulting Polonios (a fishmonger was a lowly, smelly personage in Hamlet's time), and also saying that Polonios is "fishing" for answers from Hamlet.  This also makes Hamlet look like a madman, in that he does not recognize Polonios, whom he knows so well.

HAMLET: Then I would you were so honest a man.

There are several puns in "honest".  Polonios is trying to get information from Hamlet on the sly, so that makes him, in a way, "dishonest".  Also, the wily courtier to the corrupt king does not take kindly to being told he is not as honest as a humble trader like a fishmonger.  There is also the sexual pun, in that Hamlet may be in love with his daughter -- sexual virtue in a woman was called "honesty".