‘A horse, a horse, my kingdom for a horse’ is one of Shakespeare’s best known lines. The king spoke the line in Act V of the play Richard III, after losing his horse in battle.

More generally the meaning of the expression is that the speaker is in great need of a particular item and is willing to trade something of great value to get it.

The quotation is sometimes now repeated ironically when someone is in need of some relatively unimportant item – offering their ‘kingdom’ for, a cup of tea, or a warm coat, or whatever.