1. An "unlucky" coin found on the ground. "Unlucky" or not, your amount of money was just incremented.
2. In reference to a "dime": a woman whose body is a ten but whose face is debatable.
3. Doggy style.
1. I found a quarter on the ground. It was tail side up, but that didn't change me buying a soda.
2. Girl's a dime, but tail side up.
3. I'm 'bout to get at that bitch tail side up.
by MyBoxHasInternet March 30, 2011
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"Tails up!" is a British expression sadly fallen into disuse. It was used to encourage people to high hearts and healthy optimism. Check idioms.thefreedictionary.
"Off you go lads, and tails up!" "We'll go to court with our tails up!"
by lunginghippo April 7, 2019
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