In act 2, scene 2, Hamlet, pretending to be mad, says to Polonius,

O Jephthah, judge of Israel, what a treasure hadst thou!

In the book of Judges, Jephthah, a judge or leader of Israel, made a vow to God that if he won his battle over the Ammonites, he would sacrifice as a burnt offering what first came through his door when he got home. He won the battle, but unfortunately, what first came through his door was his daughter. He felt he had no choice but to sacrifice her. Hamlet is accusing Polonius of being as willing to sacrifice Ophelia to his ambitions as Jephthah was his daughter. He is also suggesting that he knows how controlling Polonius is toward Ophelia and the extent to which Ophelia will sacrifice herself to his needs. In act 1, scene 3, Polonius points out to her that she must behave first and foremost in way that "behooves" his daughter and must give him information, which she does:

You do not understand yourself so clearly

As it behooves my daughter and your honor.

What is between you? Give me up the truth.

Jephtha is an allusion to the book of Judges in the Old Testament.  Jephtha was a military leader, and in one particular battle, he vowed that if he were successful, he would sacrifice to god the first person who greeted him at his return home.  The person who greeted him just happened to be his daughter, but the distraught Jephtha kept his word.  When Hamlet calls Polonius Jephtha, he is implying that Polonius is using his daughter to advance his own career.  And, indeed, Polonius is using his daughter Ophelia.  He shares the love letters she received from Hamlet with Claudius, and plots to use Ophelia to uncover the nature of Hamlet's madness--all to curry favor with the king. 

Hamlet also calls Polonius a "fishmonger."  Although the second meaning of this term is disputed among critics, many believe that "fishmonger" was another word for pimp.  Again, Hamlet is implying that Polonius is using his daughter selfishly.