I recently came across a website ridiculing undergraduates for turning in written work in which they had conflated the words 'organism' and 'orgasm'. But this might not have been so much the usual sign of subliteracy, nor even a Freudian slip (if ever such a thing existed); it might, just might, instead have been a sign of incredible erudition.
In Jean-Baptiste Lamarck's La Philosophie zoologique of 1809, the latter term appears to be taken as more or less synonymous with the former. Chapter 4 of the treatise, entitled 'Of Orgasm and Irritability', defines orgasm as "the state preserved by the internal, soft parts of animals insofar as they are alive." Further on he describes it as sensibilité latente, and thus as a notion that is complementary to irritabilité, which is the capacity of the body to sense and react to external stimuli.
'Orgasm' in today's sense is something like a reduction of what Lamarck meant to only its most extreme instance. Somewhere also I remember Leibniz saying earlier, in the 1670s, that the reproductive act in animals is the summum or culmination of its existence as a sensitive being, and a sensitive being is just what an animal is in its very essence. So the idea is that the sensation involved in reproduction is the ultimate expression of what it is that an animal does qua animal. So it is not at all hard to see how we slide from Lamarck's orgasme to our orgasm(s).
Interestingly, as I and others have argued in more scholarly spaces, a similar shift happens in the history of the term 'organism', which starts out describing an abstract state, like Leninism or Judaism, and ends up being a count noun that can easily be rendered in the plural. One might of course speak of 'multiple Leninisms', but one has to strain to do so, whereas multiple organisms or orgasms are easily tallied.
Anyhow, I don't know when the semantic shift occurs precisely, and would be pleased to learn some more details. I expect the occurrence of Lamarck's technical term in the title of this post will, given the way search engines work, garner more readers for it than I am used to having. But alas these will probably not be readers interested in helping me to fill in gaps in my knowledge of the history of science.