Out of the many tropes and trends to come out of anime and manga, there’s one I find especially curious: the casual use of “sadist” and “masochist” to describe characters. It’d be more understandable if it was limited to more sexually charged series, or to describe villains as “sadistic bastards,” but it occurs in just about everything—romances, kids’ shows, sports/competition series, and so-on. You see the letters “S” and “M” thrown around by characters as if it’s the most normal thing to say in a conversation.
In series like Prison School, “S&M” is used conventionally to refer to kinks and fetishes. In other cases, like Chihayafuru, the phrase is more removed from an explicitly sexual context, and could potentially be seen as simply referring to a non-sexual pleasure derived from inflicting or receiving pain. Or perhaps that layer of sexual tension and mild eroticism that permeates many anime and manga also trickle down into the ones that aren’t like that. The same could perhaps be extended to phrases like “siscon,” though many recent anime have gone out of their way to make that particular phrase anything but innocent.
I’m not against this trend of using “S&M” terminology, or at least find no need to take umbrage with it, but it really makes me wonder where the heck it all comes from. Is it a few famous titles? Could it be from some visual novels that got big among otaku? Or maybe it’s from something more mainstream, like classic Japanese literature. Yet, try as I might, any attempts at cursory research turn up fruitless. I get the feeling that there’s no straightforward answer, and that it might be bits and pieces of both Japanese domestic and foreign imported culture mixed together into a complex stew.
If anyone has any expertise on this matter, or knows any potential resources that could point me in the right direction, I’d love to know. This is one mystery that I really want to solve.
OGIUE MANIAX 
So I’m not an “expert,” but I do live in Japan and have encountered this question from friends in real life. It’s definitely mainstream and has little to do with anime or otaku exclusively. Like you said, people usually remove S/M from any sexual connotations, though comedies will often sneak in BDSM gags to be a little risque. Instead, it’s used to summarize a person’s personality: S broadly means assertive and M broadly means submissive.
I don’t know the origins of the S/M split, but it fits into the general Japanese trend of using nonsense like bloodtypes, numerology, and both western and eastern zodiac signs to determine personality. Maybe a personal example will illustrate how seriously people take this stuff. Once, when I told a nurse that I didn’t know my blood type (or star sign, or zodiac year, or S/M letter…) she practically insisted that I go get tested *right now.* She joked that she would draw my blood herself if she had to. Not for any medical reason mind you, just to determine my personality type (instead of just like… talking to me? Whatever.)
Anyway, it’s mainstream though not particularly common and, unlike stuff like bloodtypes, has the useful benefit of actually describing a loose dimension of personality. Divorced from the sexual context in the original English, it’s usually pretty innocent even if it just sounds bizarre to native speakers (like so much of Japanese-English). That’s about it really. There isn’t much to understand. It’s as simple as the dichotomy itself.
Here’s a more focused and comprehensive description for extra information:
https://www.japanesewithanime.com/2018/01/do-s-meaning.html?m=1
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I have seen the “S & M” individually and together used to explain
character traits. This may be due to Tezuka’s more explicit work
but also due to pervasive pornography in Japan where the use
of torture and allowing oneself to be tortured are portrayed.
Also this moved into yaoi stories where the characters are
stereotypically portrayed as submissive or dominant to the
point of rape. This is because it is used an an analog to
the expected “future” sexual relationship in adult life.
Thus we have the submissive Madarami striving to be seen
as dominant though all the girls and even the transvestite
mangaka agrees.
And by now we all know what happens to “rami-sempai” in
the unauthorized sequel by the same author.
Reference: read “Genshiken” and Genshiken Nidame
aka (2nd Season) and the unauthorized sequel by the original
author, “Spotted Flower”.
bliss
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