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?
Leftists have always despised liberalism - and for pretty damn obvious reasons for anyone that has a basic understanding of what left and right actually means. Liberalism is a right-wing ideology that upholds capitalism, colonialism and white supremacism, all while having a recurring habit of enabling and supporting fascism at the drop of a hat when the precious liberal order is threatened by change from below.
So yeah - as far as leftists are concerned, the term liberal is as much of an insult as the term fascist is. There is even an old leftist saying, “kill the liberal inside your head” which sums up pretty nicely what leftists think of liberals and liberalism.