"buffalo soldier" means: An African-American soldier in the US Army, serving in one of a number of segregated units under white officers, in the period after the US.... This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.