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1.
orignally to dog bites, now means that once somebody has gotten hurt or had something go wrong will be afraid to try it again (especially in love)

If you fall in love with someone and that someone rejects or hurts you-once bitten-you may be very, very reluctant-twice shy-to let yourself fall in love again.
elle was once bitten twice shy since her last relationship.
by vitalia November 27, 2006