That's all very interesting but I don't think it has anything to do with if the phrase "master" is racist.
A quick search reveals the following English idioms:
master master plan master of ceremonies
master key master class master baker
master of arts baggage master past master
grand master zen master puppet master
master of fine arts master of science master sergeant
chess master old master master builder
master bedroom harbor master master of literature
master copy master cylinder master at arms
master of library science sailing master master race
question master master of arts in library science master hand
master file master of divinity master of theology
master mason master of architecture master of arts in teaching
master of education property master master switch
drill master master lease master in business
master of the rolls master cast ballet master
master in public affairs master mechanic master stroke
master vibrator master chief petty officer master of laws
riding master harbour master job master
rattlesnake master assay master master notes
master of science in engineering master and servant wreck master
dance master games master kong the master
master plans bus master master in business administration
master mind master tape senior master sergeant
station master the master master slave
mint master special master wagon master
forage master jack of all trades and master of none master in chancery
master keys quarter master three master
two master be own master been own master
dock master dutch master fencing master
master electrician master of the horse writing master
bush master corrected master cast gang master
is own master master classes master gland
master lode master switches master tap
master two step exercise test
A quick google search on the origin of the word master comes up with this (and in fact, I don't know why you didn't search this originally).
"Master comes from the Latin adverb magis (“more”). It first appeared in English over a thousand years ago, referring to people who had authority over others, whether as rulers, employers, teachers, or fathers."
I would imagine slaves and slave owners would be in that list as well.
I'm not saying that "master" is always racist, but in certain cases, it seems pretty clear -- as in the Master/Slave J/K flip flop. The example given "master bathroom" surely seems like it could have racist connotations.
A quick search reveals the following English idioms:
master master plan master of ceremonies master key master class master baker master of arts baggage master past master grand master zen master puppet master master of fine arts master of science master sergeant chess master old master master builder master bedroom harbor master master of literature master copy master cylinder master at arms master of library science sailing master master race question master master of arts in library science master hand master file master of divinity master of theology master mason master of architecture master of arts in teaching master of education property master master switch drill master master lease master in business master of the rolls master cast ballet master master in public affairs master mechanic master stroke master vibrator master chief petty officer master of laws riding master harbour master job master rattlesnake master assay master master notes master of science in engineering master and servant wreck master dance master games master kong the master master plans bus master master in business administration master mind master tape senior master sergeant station master the master master slave mint master special master wagon master forage master jack of all trades and master of none master in chancery master keys quarter master three master two master be own master been own master dock master dutch master fencing master master electrician master of the horse writing master bush master corrected master cast gang master is own master master classes master gland master lode master switches master tap master two step exercise test