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.