A person or thing which initially displays promise, but then fails to live up to that promise. After British singer/song-writer Pete Doherty of Babyshambles, ex of The Libertines. A twat.
"A young lady behind whom we were merrily strolling had a marvellous behind and spectacular legs. Our expectations were thus raised over the prospect of seeing her from the front. Sadly, upon passing said filly, her breasts proved to resemble spaniels' ears and her face a bag of hammers. What a Pete Doherty."