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Meaning of baker's dozen

bak'er's doz'en

Pronunciation: [key]
  1. a group of 13; a dozen plus one: from the former practice among bakers and other tradespeople of giving 13 items to the dozen as a safeguard against penalties for short weights and measures.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.