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Meaning of double-nickel

dou•ble-nick•el

Pronunciation: (dub'ul-nik'ul), [key]
— n. Slang.
  1. the national speed limit of 55 miles per hour as established in 1974 on U.S. highways.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.