This is a very old and completely obsolete term.
'Hottentot' is a politically incorrect term for the nomadic peoples of South Africa. The most commonly known of these groups today are the !kung and the San. You may also have heard the term Bushmen (which is also politically incorrect). Regardless, they all speak some branch of the Khoisan language family. These languages all have a number of clicks and pops (the ! in !kung indicates one of the tongue clicks). These languages were long viewed as incomprehensible (and the fact that the natives tended to walk about mostly naked didn't help their image).
The term hottentotism was coined by John Conrad Amman, who is best known for working to teach deaf-mutes to speak in the late 1600s and early 1700s. He also worked with severe stutterers, and the most severe of these, whose stuttering was so severe as to make them unintelligible, he referred to as having hottentotism.
It is perhaps worth noting here that those who stutter will often develop escape behaviors -- a muscle twitch or vocal habit that will help them break their 'block'. These may be quite extreme, such as kicking out, grimacing, smacking lips, and making tongue clicks. It is possible, although not certain, that Amman worked with some stutterers that used these sounds, particularly glottal stops, tongue clicks, and lip smacks, which might have lead him to use the term hottentotism. Or maybe he simply couldn't think of any form of speech more degraded than that spoken by the heathens of Africa.
It has since been recognized that 1) we shouldn't name medical disorders after ethnic groups and 2) if you just use simple words like 'mild', 'moderate', and 'severe' people will still respect you as a scientist. So these days hottentotism would simply be referred to as a severe stutter.
Although the word Hottentot is these days considered non-PC and well on its way to being archaic, you may also find, occasionally, this term used to refer to either:
1. Any word borrowed from the Hottentot language. These would include karoo and gnu.
2. Theft. As the San traditionally had an underdeveloped sense of personal property (from the European viewpoint), they often assumed that the people who had a lot of stuff would naturally share with those that had next to nothing -- much to the European explorers' distress.