A piece of a millstone - רכב פלח pelach recheb, a piece of a chariot wheel; but the word is used in other places for upper millstones, and is so understood here by the Vulgate, Septuagint, Syriac, and Arabic.
And all to break his skull - A most nonsensical version of גלגלתו את ותרץ vattarits eth gulgolto, which is literally, And she brake, or fractured, his skull. Plutarch, in his life of Pyrrhus, observes that this king was killed at the siege of Thebes, by a piece of a tile, which a woman threw upon his head.
The phrase “all” to is now obsolete, and means “quite,” “entirely,” as in Chaucer, Spenser, and Milton.