Mom Says The R-Word Is OK Because It's In The Dictionary

[Irish_Dem]

22. Yes

With the introduction of the intelligence test, developed by Binet in France, and brought to this country by Goddard, it became an accepted practice to relate these three terms to specific I.Q. scores *—idiot for those scoring below 25, imbecile 25 to 50, and moron 50-70/75.

These terms were acceptable and standard usage at the time. But of course people grabbed onto to the nomenclature
as pejoratives, so then the ratings went to low average, borderline, very low, etc.