Abbott's is a brand of bitters that is no longer made commercially, but there are a number of classic cocktails that still call for the stuff. If you're looking to recreate it, this recipe is a reverse-engineering effort of it, and is by no means absolute. Try variations to suit your needs!
Here are some other sites that describe Abbott's Bitters:
Reverse engineered recipe on Art of Drink (http://www.artofdrink.com/bitters.php?ingredients=Abbotts+Bitters)
Mark Sexauer's Cocktail blog entry on Abbott's Bitters (http://marksexauer.wordpress.com/2008/09/07/abbotts-bitters-kinda/)
Abbott's Bitters entry in the Internet Cocktail DB (http://www.cocktaildb.com/ingr_detail?id=14%20)