For the longest time, “liberal” seemed like it basically just meant “Democrat” the same way “conservative” has/had been used to mean “Republican.” Now, it seems like it means “bad Democrat” and is even worse than being MAGA the way many seem to use it. Where did its use as an insult within the [relative] political left come from, and what does it specifically accuse/identify someone of/as?
Liberalism is a right wing ideology, which because America is so right shifted is seen as left in their country or at least a counterpoint to conservatism when they’re basically the same thing - deregulate, don’t tax, “free market” capitalist nonsense that’s destroying the planet.
So from a left wing perspective it’s a derogatory term. Most leftists take on a more worldly understanding, so even American leftists will use it this way.
As for American right wingers, they’re just using it for hate. I doubt they could even explain who they mean.