The meaning and origin of the expression: I will wear my heart upon my sleeve

I will wear my heart upon my sleeve

What's the origin of the phrase 'I will wear my heart upon my sleeve'?

From Shakespeare's Othello, 1604:

IAGO:
It is as sure as you are Roderigo,
Were I the Moor, I would not be Iago:
In following him, I follow but myself;

Heaven is my judge, not I for love and duty,
But seeming so, for my peculiar end:
For when my outward action doth demonstrate
The native act and figure of my heart
In compliment extern, 'tis not long after
But I will wear my heart upon my sleeve
For daws to peck at: I am not what I am.