A structure built by stacking playing cards with a pyramid shape, used as entertainment or a metaphor for something extremely fragile and unstable.
From 'card' (playing card from French 'carte') + 'castle' (from Latin 'castellum'). The compound emerged in English sometime in the 1600s as card games became popular entertainment.
A 'house of cards' or 'card castle' is the perfect metaphor because it's impossibly fragile—one breath and it collapses—so English speakers use it to describe any plan that looks impressive but could fail instantly.
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