The phrase get off my lawn, in popular culture, is the cliché thing that old people say to rowdy teenagers. It also has a more metaphorical meaning as a phrase indicating that its speaker is stubbornly and perhaps prejudicially unwilling to deal with inevitable changes to a world that he no longer feels comfortable in.
Speaking of which:
Trump is right that ejecting people with potentially legitimate asylum claims from the country without due process would, in fact, be a violation of U.S. law, but he’s wrong about the U.S. being the only country that has such rules—the right to fair hearing for asylum is a principle of international human rights law as well.
As it happens, there is actually a Clint Eastwood movie—Gran Torino—about Clint Eastwood telling immigrants to get off his lawn, except in the movie, Clint Eastwood’s character ultimately realizes that many immigrants are good people, which seems unlikely to happen in real life here.