To cost so much money that it causes financial hardship or exhausts one's resources. Originally meant to win all the money available, but now typically means something is very expensive.
This phrase originated in gambling houses of the 16th century, where 'breaking the bank' meant winning all the money the house had set aside for a particular game. The 'bank' was literally the dealer's fund of money available for that gaming table.
The phrase underwent a complete semantic flip over the centuries - it went from meaning 'win everything' to 'lose everything' or 'spend everything.' This reversal happened because people began using it from the spender's perspective rather than the winner's perspective, making it about depleting your own resources rather than depleting someone else's.
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