Probably from engineering in the 30s/40s, I'd bet. Say you had two gun turrets with servos, and used a synchro slaved to another synchro to have one track whatever the other was pointing at.
It's pretty obvious terminology for systems that don't support dynamic hierarchy changes.
The term master does have origins in the latin word "magister" (teacher, leader, chief or someone with a license to each philosophy or liberal arts). The middle english form, maister, was not connected to slavery either, it was more about someone of authority (ie, lord, ruler).
A lot of times it's use similarly like "Master Clock". The term in technology is strongly connected to that root; the master in a protocol is the leading participant.
I think simply not using "slave" for the passive or accepting device would be sufficient to reduce the problem since that is the word that has even in the deeper roots of it's origin the same meaning as today. (Slave comes from Slav, a group of people enslaved in the medieval ages and later expanded to cover all "slavs")
For a current project I'm using master/client as terminology.
It's pretty obvious terminology for systems that don't support dynamic hierarchy changes.
EDIT: Consulting some docs from the time ( http://www.eugeneleeslover.com/USNAVY/CHAPTER-10-D.html ), this may not be the case. I'm genuinely curious now.