The term antifa is pretty common among anti-fascists in Germany at least. Maybe it hasn't catched on much in the US because the country has been busy sleepwalking into totalitarianism.

"Antifa" branding was embraced by anti-fascist soldiers of WWII. That's a bit of a while ago now!

My first experience with it was when I learned a couple of decades ago that not all skinheads were nazis and that there were in fact anti-fascist skinhead punks who called themselves "Antifa." To be honest, some of them openly bragged about beating up nazi skinheads so I suppose there was some violence involved. I wouldn't be surprised if Trump heard about those folks and they're the ones he means, even though they don't totally own the idea of "Antifa."

From Wikipedia,

The English word antifa is a loanword from the German Antifa, where it is a shortened form of the word antifaschistisch ("anti-fascist") and a nickname of Antifaschistische Aktion (1932–1933), a short-lived group which inspired the wider antifa movement in Germany.[43][44][45] The German word Antifa first appeared in 1930. The long form antifaschistisch was borrowed from the original Italian anti-Fascisti ("anti-fascists").[43

...

In 2007, Rose City Antifa, likely the first group to utilize the name antifa, was formed in Portland, Oregon by former ARA members.[81][5][82] Other antifa groups in the United States have other genealogies. In Minneapolis, Minnesota, a group called the Baldies was formed in 1987 with the intent to fight neo-Nazi groups directly.[61] In 2013, the "most radical" chapters of ARA formed the Torch Antifa Network,[83] which has chapters throughout the United States.[84] Other antifa groups are a part of different associations such as NYC Antifa or operate independently.[85]

...

The American antifa movement grew after Donald Trump was elected president of the United States in 2016. Antifa activists' actions have since received support and criticism from various organizations and pundits. Some on the political left and some civil rights organizations criticize antifa's willingness to adopt violent tactics, which they describe as counterproductive and dangerous, arguing that these tactics embolden the political right and their allies.[13] Both Democratic and Republican politicians have condemned violence from antifa.[14][15][16][17] Many right-wing politicians and groups have characterized antifa as a domestic terrorist organization, or use antifa as a catch-all term,[18] which they adopt for any left-leaning or liberal protest actions.[19] According to some scholars, antifa is a legitimate response to the rise of the far right.[20][21] Scholars tend to reject an equivalence between antifa and right-wing extremism.[2][22][23] Some research suggests that most antifa action is nonviolent.[24][25][26]