I'm not 100% sure what you're referring to, though I suspect it's to the disease of "drapetomania." In 1851, an American physician by the name of Samuel Cartwright was appointment chairman of a committee of the Louisiana State Medical Convention to examine diseases peculiar to black slaves. This was a period where psychological disorders of single, impulsive behaviors were coming into vogue -- think pyromania or kleptomania. Cartwright firmly believed that slavery was the natural state for blacks (tied apparently to Biblical thought and the Mark of Canaan -- hopefully someone more knowledgeable can chime in here). If the natural state were to be enslaved, therefore, a desire to run away would be a form of a mental disorder. And yet -- slaves did run away. Cartwright defined this new condition at drapetomania, from two Greek words meaning "a runaway slave" and "mad or crazy". He also defined a parallel condition, "dysaesthesia aethiopia," defined in his words as the tendency of Africa slaves "to be idle, to wallow in filth, and to indulge in improper food and drinks."

It would be a mistake to think that either of these diagnoses were ever taken seriously, even in the South. The definition was published in the New Orleans Medical Journal in 1851, but afterwards pretty much every time these appear in print, the intent is to mock it, like in Francis Law Olmsted's The cotton kingdom: a traveller's observations on cotton and slavery in the American slave states.

You can read the original case description if you want, reprinted at PBS.