"Our sagest men of lore define
The kingly state as masculine,
Paiseant, martial bold and strong,
The stay of right, the scourge of wrong;
Hence those that England's sceptre wield,
Must buckle on broad sword and shield,
And o'er the land, and o'er the sea,
Maintain her sway triumphantly."
This, unfortunately, is only one side of the question: and, though satisfactorily accounting for the shape of the shield of royalty, does not enlighten us on the "origin and meaning" of the lozenge.
Barrington, Display of Heraldry, 1844:—