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.

A portmanteau of carpe diem, a Latin phrase that means “Seize the Day!” and DM, which stands for Direct Messages. This word means seizing the day via DMs.

This is an abbreviated sentence:

Once your are bitten by a dog, you will be twice as shy about approaching that same dog again.

Metaphorically, the concept applies to more than just dogs. Such as cats.