The use of the term "master-slave" is currently quite common in technical descriptions of control relation between two devices: automotive clutch and brake systems (master cylinder, slave cylinder), clocks, flip-flop circuits, computer drives, radio transmitters, and others. This essay describes the history of its technical use, dating from its origin in 1904, and the various relations between its technical usage and its racialized social connotations. We then examine various hypotheses for why a morally objectionable analogy became so popular, comments by African American engineers both for and against its continued usage, and some recommendations for altering its usage in the future.