Es como un piedrazo en la cabeza.
[caption 26, Verano Eterno > Fiesta Grande > Part 6]

That's gotta hurt. In Spanish, the suffix azo can signify a blow by the object at the root of the word. So, piedrazo means a blow by a piedra, or stone. By this logic:

Bala -> "bullet"
Balazo -> "blow by a bullet; a gunshot wound."

Codo -> "elbow"
Codazo -> "blow by an elbow; nudge."