About the year 2000, I started to notice the English words yes/yeah and no changing their meanings. People in the UK used these as filler words (often preceded by but) to keep their mouths busy when they couldn't think of what to say, as are term like well and you know. At the extreme, people would babble out gibberish like this during lulls in conversation: But yeah. No. Yeah.
People also started answering yeah to questions that weren't yes/no, as:
Speaker A: What did you think to that? Speaker B: Yeah, it was good.
This suggest speaker B answered as if speaker A's question yes/no, maybe Did you like that? This use of yeah isn't a terrible sin, but soon I heard people using no to convey the same positivity, which seems a contradiction.
Finally I heard simple inversion of meaning: yes/no questions were answered with no, even though the meaning was obviously positive.
Speaker A: Did you like that? Speaker B: No, it was good.
From the start I heard these things in the mouths of educated speakers. Below are some examples.
Kearney: But I guess from the point of view of um firms here in in in London, the City of London, they'll be looking at him, he's a former Rothschild banker, he understands their world. Harrison: No absolutely this is the thing you know culturally and uh you you you know in terms of his outlook he's gonna feel very familiar.
Source: THE WORLD AT ONE, BBC Radio 4, 2017-May-08, 13:00-13:45
Al-Khalili: Does that mean then if causal set theory is right, there's no need for dark energy? Or am I taking it a step too far? Dowker: No that's right. So it replaces... so yeah, dark energy is just the the placeholder name that we give to this phenomenon I suppose so yeah so...
Source: THE LIFE SCIENTIFIC, BBC Radio 4, 2017-May-09, 09:00-09:30
I accept that languages change. But to use a word for the very opposite of its usual meaning is a step too far. I despair for humanity that it can commit such perversions.