Will all great Neptune's oceans wash this blood clean from my hand

To remember this quote use the following mnemonic.

Will all the great Neptune's oceans please reveal yourself.

Now take it in turns to wash this blood from my hand.

"Will all great Neptune's oceans wash this blood clean from my hand." Act 2, scene 1, line 63.

Macbeth is an experienced warrior and is used to killing but he comes back to his chambers badly shaken from murdering his king. Seized by deep revulsion he says the above quote to Lady Macbeth. 

Shakespeare used hyperbole to express Macbeths deep horror and regret after murdering King Duncan. 

The metaphor of Neptune's oceans suggests that no amount of water will ever remove the "stain" of regicide.