Meaning of heads or tails

heads' or tails'

Pronunciation: [key]
  1. a gambling game in which a coin is tossed, the winner being the player who guesses which side of the coin will face up when it lands or is caught.
  2. the tossing of a coin in this manner to determine a question or choice.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.