As the internet grows, the evolution and the death of language will only get faster.
I had already seen people claiming that 「哇塞」 was a Taiwanese swear, with article after article swearing they had evidence. Now even Baidu Baike and AI search say it began as a crude Taiwanese verb for sex.
But I grew up in southern Taiwan. As a child I never heard the people around me say this word. I heard it constantly from people who spoke Mandarin. The phrase is Mandarin through and through, not Taiwanese.
Taiwanese does have swears built on 塞 XXX, but that usage came much later — mainly after the genital swear 「雞掰」 had already been turned into 「機車」.
Because 塞 is the Taiwanese word for “to drive,” riding a scooter (機車) was given a sexual meaning. Only then did 塞 come to mean sex, as a stand-in for 「幹」.
哇 is “I” in Taiwanese, so 哇塞恁娘 means “I fuck your mother.”
It only sounds similar to 哇賽. It is not the same word. That Taiwanese usage appeared around the mid-1980s. The Mandarin 哇賽 I was already hearing all the time in the 1970s.
The Chinese domestic internet now holds the largest share of Chinese-language content online. Falsehoods that have hardened into “facts” are everywhere. Trace what can still be found of this etymology, and the earlier pieces all appear after 2009, on that domestic internet. The source is a paper by someone named Hou Yinmei, an associate professor at Shangqiu University, and a feminist. I have read several of her papers. They are sloppy: full of speculation and prejudice.
She is also a promoter of Mandarin, and the smearing of dialects comes easily to her. In this online environment the public cannot easily test a claim. In the end the error really did become the accepted fact.
In Taiwan this piece of etymology was brought in by feminist groups. A younger generation of Taiwanese now takes it as the truth.
When the people who still hold these memories are gone, everyone will think their version was the true one.