What is a famous idiom from Hamlet by William Shakespeare?

There are several idioms from Shakespeare's Hamlet that have become famous.  One idiom was spoken when Hamlet spoke to the ghost.  The ghost told him,

 "I could a tale unfold whose lightest word 

 Would harrow up thy soul, freeze thy young blood, 

 take thy two eyes, like stars, start from their spheres, 

 Thy knotted and combined locks to part 

 And each particular hair to stand on end, 

 Like quills upon the fearful porpentine."

The ghost...

There are several idioms from Shakespeare's Hamlet that have become famous.  One idiom was spoken when Hamlet spoke to the ghost.  The ghost told him,

 "I could a tale unfold whose lightest word 

 Would harrow up thy soul, freeze thy young blood, 

 take thy two eyes, like stars, start from their spheres, 

 Thy knotted and combined locks to part 

 And each particular hair to stand on end, 

 Like quills upon the fearful porpentine."

The ghost was referring to a frightening tale.  This tale was so horrifying that it would make someone's "hair to stand on end."  In writing this, Shakespeare was referring to how a person gets goosebumps and how the hair on their skin stands up when they are frightened.  

Another well known idiom is about the "primrose path."  Ophelia spoke to Laertes about her romantic interest in Hamlet.  Laertes thought that she should forget about such an idea.  Ophelia told him,

"Do not, as some ungracious pastors do,

Show me the steep and thorny way to heaven,

Whiles, like a puff'd and reckless libertine,

Himself the primrose path of dalliance treads."

The "primrose path" was referring to an easy life, or one of leisure.  Ophelia was accusing Laertes of hypocrisy in telling her what to do about Hamlet while he himself did not exercise such caution in his own life.