pacuEnglishOxford New American Dictionarypacu /ˈpako͞o, , ˈpæku , paˈko͞o, pæˈku/ ▸ noun (plural same) a deep-bodied, herbivorous freshwater fish native to northern South America. Colossoma nigripinnis, family Characidae. – ORIGIN early 19th century : from Tupi pacú.