Rehoboth Basters

[gusl]

while reading about Namibia's ethnic groups, I came across:

Basters
The Basters are mainly the combination of indigenous Khoisan people and European white settlers. They are also referred to as coloureds. The Baster community consists of between 50 to 60 000 people who speak Afrikaans and at their own request are registered as Rehoboth Basters.

Try to guess the etymology!