The multitudinous seas incarnadine
[Knocking within] Macbeth:
Whence is that knocking?
How is't with me, when every noise appalls me?
What hands are here? Hah! They pluck out mine eyes.
Will all great Neptune's ocean wash this blood
Clean from my hand? No; this my hand will rather
The multitudinous seas incarnadine,
Making the green one red.
"The multitudinous seas incarnadine" is understandably confusing to modern readers, but Macbeth explains his meaning in the following line. Shakespeare makes a verb out of "incarnadine," a sixteenth century adjective meaning "pink." (The Latin root carn-refers to flesh, and thus, in its derivatives, to flesh color.) "To incarnadine" is thus to turn something pink or light red—what Macbeth imagines his bloody hands will do to Neptune's green ocean [see A SORRY SIGHT]. After Shakespeare, the verb and adjective have both come to refer to the color of blood itself—crimson—rather than to the light red of a bloodied sea.
Macbeth has come to recognize that his guilt can never be washed off, even if the blood can be washed from his hands. Instead, his guilt will poison the world around him, which he compares to an ocean. He has already begun to hallucinate: here, he imagines hands plucking out his eyes in retribution for the murder of Duncan.