Why isn't RMS called SMR, when it's (linguistically) a square mean root?
In the language of functions, it's just fine as it is.

If we have a waveform, we first need to square it. 


  S = square(waveform).


Then we need to average the result. 


  MS = mean(square(waveform))


Then we need to take the square root of that. 


  RMS = root(mean(square(waveform)))


Do bear in mind that both conventions are used for whether the operator comes before or after the parameters, neither is 'correct', they are conventions. A lot of purists like reverse Polish notation, seemingly just for the lols. When we go to compute RMS from digitised waveform data, we are usually using a programming language, for which this functional notation is natural. And it's always been called RMS. If you start calling it SMR, nobody will know what you mean, and they'll think you unnecessarily pedantic and strange if you try to insist.