The term that slave owners called African American slaves in the 1800s that starts with the letter "N" is "Ni***r". It is an offensive and derogatory term that was used to dehumanize and degrade African Americans during the time of slavery and beyond. The term is now widely considered to be unacceptable and highly offensive in modern times. It is important to recognize the historical context and harm associated with such language and to strive to promote more inclusive and respectful language in all aspects of society.