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?
I think others have answered your question better than me, but I’ll chip in my two cents anyway.
The definition depends on who is saying it.
Within mainstream US politics, Republicans use “liberal” as a catch-all pejorative for any person or group further to the left of themselves. It is usually aimed at Democrats but could also refer to Greens, communists, etc.
The irony is that, in a broader political context, Republicans are very much liberals, too. People outside the US political mainstream who sneer about “liberals” are usually referring to this larger group, which basically encompasses the capitalist status quo in the “western” world.