'Tis sweet and commendable in your nature, Hamlet,
To give these mourning duties to your father:
But, you must know, your father lost a father;
That father lost, lost his, and the survivor bound
In filial obligation for some term
To do obsequious sorrow: but to persever
In obstinate condolement is a course
Of impious stubbornness; 'tis unmanly grief;
It shows a will most incorrect to heaven,
A heart unfortified, a mind impatient,
An understanding simple and unschool'd:
For what we know must be and is as common
As any the most vulgar thing to sense,
Why should we in our peevish opposition
Take it to heart? Fie! 'tis a fault to heaven
,A fault against the dead, a fault to nature,
To reason most absurd: whose common theme
Is death of fathers, and who still hath cried,
From the first corpse till he that died to-day,
'This must be so.' 


Cain and Abel
In the book of Genesis in the Bible, Cain and Abel are the sons of Adam and Eve. Cain was a farmer, Abel a shepherd. When both brothers made sacrifices to God, He preferred Abel's offering, and Cain killed him. This was the first murder, and Abel, therefore, the 'first corpse' mentioned in this episode's portion of the text. Cain was thereafter punished with a lifetime of wandering, and with 'the mark of Cain', a sign from God that prevented anyone from killing him - perhaps as a warning not to commit his sin again. 

Unmanly grief
The question of what it means to be a man (and what it means to be a woman) crops up repeatedly in the play. Here's the first instance of the issue, with Claudius's dressing down of Hamlet in his grief.