Be Careful Calling People 落后: While you can say someone's thinking (思想) is
落后, directly calling a person
落后 can be very insulting. It implies they are unsophisticated, provincial, or even uncivilized. It's much stronger and more offensive than calling someone “old-fashioned” in English. For fashion or style, it's safer to use
过时 (guòshí) - “outdated.”