The use of the English term 'scientism' echoes that of the French 'scientisme'. Both words have primarily been terms of abuse, implying the application of scientific methods beyond their proper domain. Peter Schöttler has traced the French phrase back to the 1840s. It seems to have arisen from disputes between Comtean positivists and more conservative, religious authors. Some of the Comteans embraced the label. Here, for instance, is Felix le Dantec in 1911:
I believe that science ... alone can solve all the issues that have meaning ... and I think it will penetrate to the mysteries of our emotional life and explain to me even the origin and structure of hereditary anti-scientific mysticism cohabiting with the most absolute scientism. But I am also convinced that men posed many questions that mean nothing." (le Dantec, Le chaos et l'harmonie universelle [F. Alcan, 1911])[Please excuse my linguistic feebleness, but the above rendering is my Google-assisted translation of le Dantec's French.]
Aided by Google's Ngram, James Schmidt has charted the early development of the English 'scientism', noting a spike in its use around 1920. Schmidt identifies two sources of this increase. One was a 1919 theological book by Charles Gray Shaw’s (The Ground and Goal of Human Life [reviewed in The Personalist 4 (1923): 67-9]). Shaw takes Comte and Spencer to be paradigms of scientism (Comte's name appears more than twenty times in the index of Shaw's book). Here's Shaw: 'The attempt to socialize science or to make sociality scientific has been the undoing of scientism; so that who can deny that Comte and Spencer have made scientism appear silly?' (p. 177) Shaw regarded scientism as being inimical to 'individualism'. In one of his earliest uses of 'scientism' in the above-linked book, Shaw says,
The scientism which now attempts to interpret the world for man had its beginning with the inception of modern thought; but it was not until the science of the organic world advanced beyond the study of the inorganic that scientism gained the upper hand and drove the self from the world of things. How such scientism arose and how it achieved its victory over the self, must now become the subject of analytical investigationShaw's writing abounds in ill-defined abstractions, including not only scientism but, also, its putative contraries (among which are 'aestheticism' and the 'spiritual').
Schmidt says that the other source of 'scientism's' 1920 spike was an article by Charles Cestre on the plight of labourers. Cestre's use seems idiosyncratic, though he, too, sees a tension between scientism and individualism. (Here's Part II of Schmidt's history of 'scientism'.)
Among the most prominent uses of 'scientism' before 1950 was that of Friedrich von Hayek. In his 1942 paper, 'Scientism and the Study of Society. Part I', Hayek introduced the term as follows:
To preclude any misunderstanding on this point we shall, wherever we are concerned, not with the general spirit of disinterested inquiry but with that slavish imitation of the method and language of Science, speak of 'scientism' or the 'scientistic' prejudice. Although these terms are not completely unknown in English, they are actually borrowed from the French, where in recent years they have come to be generally used in very much the same sense in which they will be used here. It should be noted that, in the sense in which we shall use these terms, they describe, of course, an attitude which is decidedly unscientific in the true sense of the word, since it involves a mechanical and uncritical application of habits of thought to fields different from those in which they have been formed. The scientistic as distinguished from the scientific view is not an unprejudiced but a very prejudiced approach which, before it has considered its subject, claims to know what is the most appropriate way of investigating it. (Hayek, 'Scientism and the Study of Society. Part I' Economica 9 : 269 [italics added])I've omitted Hayek's three footnotes from the above passage. In one of them, he cites two French sources re. the use of 'scientisme'. One of them is J. Fiolle's Scientisme et Science (Paris, 1936). The other one is André Lalande's Vocabulaire technique et critique de la philosophie (4th ed.).