Percy's head

v.
1. To publicly redeem one's self through humiliation or defeat of a rival (often a rival who has bested you in some way).

n.
1. Payback or revenge.

Etymology: Thought to be a bastardization of a passage from Shakespeare's "Henry IV, part 1" in which the dishonored Prince Hal is speaking with his father about his shame and Harry Percy's (Hotspur) honor:

I will redeem all this on Percy's head
And in the closing of some glorious day
Be bold to tell you that I am your son;
When I will wear a garment all of blood
And stain my favours in a bloody mask,
Which, wash'd away, shall scour my shame with it:
And that shall be the day, whene'er it lights,
That this same child of honour and renown,
This gallant Hotspur, this all-praised knight,
And your unthought-of Harry chance to meet.