The story
From William Shakespeare's Richard III (written c. 1593; published 1597), Act V, scene iv (spelling modernized): "A horse, a horse, my kingdom for a horse."
Etymology from Wiktionary, CC BY-SA 4.0.
my kingdom for a horse
Today it means: The speaker is willing to sacrifice anything to obtain a seemingly unimportant item that is invaluable at a critical moment. Full definition →