I think I first heard of “the Red Pill” and “being Red-Pilled” about 2012 or later; though I knew it from The Matrix, a film I saw and vaguely liked back in 2001 or so.

The film dates to 1999 but Red Pill took a while to be absorbed by our societal gut:

Late 2012/early 2013 seems to have been a transitional time in the West.
As I have encountered it, Red Pill is used to indicate an unauthorized right-wing realisation, for example that genetics determine IQ or that women don’t think like men. Despite The Matrix being a popular film directed by a pair of trannies, full of anarchist and anti-authoritarian themes, I’ve never come across the term outside of dissident Right circles.
For me, the Red Pill is the removal of one layer of the grand onion. As in the film, you open your eyes and see everything in a different light.
It is not specific to ideology – it is merely the act of realising the truth, in contrast to a consensual illusion.
That perhaps explains why Red Pill is not a Leftie term – the consensus is Left-wing; just as almost all calls for censorship come from the Left, so they find no use for the concept of the Red Pill: for them, the illusion is reality. To challenge the prevailing account is to challenge the Left, because they are the convenient servants of the System.
Even in my early 20s I was essentially a man of the Right, for I preferred Beethoven to Oasis, Oxford to Bradford, Titian to Damien Hirst, as I think do many on the Left – the difference between us, that I followed my aesthetic preferences to their logical conclusion, and the champagne socialists I know advocate for mass immigration, socialist levelling, but choose to live in all-white suburbs and would prefer Rembrandt to a Tracey Emin.
My Red Pill odyssey began later.
With each shedding of the onion skin, I became estranged from my younger self and from those still plugged into the Matrix.
As Morpheus tells Neo:
The Matrix is a system, Neo. That system is our enemy. But when you’re inside, what do you see? Businessmen, teachers, lawyers, carpenters. The very minds of the people we are trying to save. But until we do, these people are a part of that system…and that makes them our enemy. You have to understand, most of these people are not ready to be unplugged. And many of them are so inert, so hopelessly dependent on the system that they will fight to protect it.
The Red Pill severs you from the NPCs. You may as well be the only player-character, everyone you once called a friend now revealed as a computer-controlled figure.
Earlier this week I was talking to a group of German women, mostly late 50s to 70s. They said that women are discriminated against everywhere, then blithely added that most schoolteachers are women. I suggested that women tend to be more interested in people than things; men, vice versa. One of these women, who earns about ten times what I’ve ever made, and is probably more intelligent than me, smirked and shook her head and said that it’s impossible to generalise because there are women who are engineers. I said, patiently, “yes, that’s what I mean, there are individuals but in general -” and she just kept smirking and shaking her head, and then adduced as incontrovertible evidence of her views, that she preferred playing with cars to dolls when she was young.
At this point I gave up and just assumed a polite facial expression.
Another of these smug women said that men are becoming like women now, and that is obviously good.
Almost all the women nodded enthusiastically.
Group X says the world would be better if everyone was identical to Group X, and then they sit there smiling at their own profound insight. And it doesn’t occur to them that they may simply be narcissistic and deluded. In a sense, it is supremely German – for all their cultural suicidal impulse, Germans assume everyone should be like them, and be made so by force if need be. The self-righteousness of German women, their finger-wagging assurance and revolting smugness and idiocy, can be hard to stomach. Arrogance attended by its constant handmaid – ignorance.
I wondered, later, if it would have been possible to offer a counter-argument. Unlikely, since I would have needed to talk without interruptions for about 2 minutes. The error in their reasoning, the inability to understand the difference between the individual and the general, was so great I felt I was dealing with NPCs running on a limited script. If someone with a good university degree, from the pre-degree-inflation years, making a good salary, cannot understand the meaning of “to generalise” (generalisieren in German), what communication is possible? Imagine saying “Spain is hotter than Finland” and a smug German woman shakes her head and then sneers: “No no no. I can tell you, I have been on holiday in Finnland in 2002 and it was 28 grad, and in Spanien at that moment only 26 grad, so you are wrong.”
So you say, “yes, but in general -”
And she says, “it is not possible to generalise. Global warming makes Finnland hot because of Donald Trump.”
So you say, “yes, but the average -”
And she says, “no no no, you cannot generalise. One winter it has been for one day 4 grad in Spanien and 5 grad in Finnland. It stands in Der Spiegel so.”
You are separated from the Blue Pilled NPCs by an insuperable, existential barrier. They may be more intelligent and make more money, be vastly more successful than you, but they cannot understand the difference between the individual and the group, cannot understand the concept of “to generalise” or “on average”. You experience intellectual isolation as the words you were taught to use to communicate fail to penetrate the arrogant ignorance of the terminally Blue Pilled. It is not a question of intelligence here, since two of the other, less intellectual women were on my side in this, and I am myself unintelligent and unsuccessful. It is a question of the Red Pill, of having begun the journey to reality.
In accepting reality, you isolated yourself from the majority of the human race. In a sense, you inhabit a different reality, by your perception. The words no longer mean anything, so “to generalise” for you means “to draw general conclusions from large data sets, regardless of individual difference” and to the Blue Pilled the very same verb means “to say that every single individual conforms to a certain description.”
Black is white and white is black. What can you do except smile politely and pretend to agree?