Break The Buck Meaning

(US, idiomatic, finance, of a money-market fund) Fall below the value of one dollar per share.[1]

Example: 2000: Charles A. Jaffe, Chuck Jaffe’s Lifetime Guide to Mutual Funds: An Owner’s Manual, page 110 (Perseus Books Group; ISBN 0738202738, 9780738202730)
  In 1994, during the Orange County bankruptcy, a few funds were poised to break the buck, until their parent companies stepped in.
2008, "Money-Market Funds Get $50 Billion Backstop From US (Update1)", Bloomberg, Sep 19, 2008
  "We were likely going to see more funds halt redemptions" and break the buck. The insurance program is part of a wider rescue package […]