This phrase was in common use in the mill city in Massachusetts where I was born and raised and was used derogitoraly to describe anyone who wasn't amenable to assimilation.
It worked because there was a common understanding that assimilating was desirable.
Learning English was top priority for most new arrivals, because they understood the importance.
My grandmother's parents, from Germany, insisted their children speak only English, because "We're in America now."
"My grandmother's parents, from Germany, insisted their children speak only English, because "We're in America now."
When my husband's family came over from Germany in the 1880's, the same applied to them. No one went up to the schools and demanded that everybody start speaking German. They would immediately have been told to find their way back to Germany. Many have simply forgotten that we are in America and we must all follow the rules of OUR constitution. Not the rules from some other country.