The word comes from outside of audio, and probably comes from manufacturing.
Originally all amplifiers were "mono blocks" but they weren't called that. THey were just amplifiers. At this time stereo was not real and 1 amplifier was all you needed. :) See if you can find some vintage tube amps made for the early motion picture theaters.
Then stereo came out (1930s per https://en.wikipedia.org/wiki/Stereophonic_sound#History) and they were just "stereos" and stereo amplifiers became the normal way of packaging an amplifier.
So the term monoblock applied to an amplifier probably came at some time after that.