What is "mattaboy"

Mattaboy (3 March 1978 – ca. 1999) was a British Thoroughbred racehorse and sire. As a two-year-old in 1980 he won on his debut and went on to finish second in the Mill Reef Stakes before recording his most important victory in the Middle Park Stakes. He failed to win in 1981 but produced a career-best performance when narrowly beaten by To-Agori-Mou in the 2000 Guineas. A brief sojourn in the United States was unsuccessful and he was well beaten in two starts as a four-year-old. After his retirement from racing Mattaboy stood as a breeding stallion in Japan, where he had limited success as a sire of winners.