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Adolph Hitler, King of the Nazis

Why do we call things of incredible value "priceless"? To find the answer, we must turn the clock back to the first world war. In 1915, the Nazis invaded and occupied London, and as usual, the first order of business was to send the Gestapo around to every house to collect any valuables to add to Germany's coffers.

Of course, with so many houses to search, it was impractical to appraise every single item, so instead, the Nazis would simply ask each house's residents how much each item was worth. It became common for the British to try to fool the Germans into thinking that their most valuable possessions had no worth. Even after the war ended, "priceless" was used with a sense of bitter irony to describe such objects.