What does it mean to be left out? What's the trick?

Cold door is a rhetorical method of metaphor, that is, metaphor is particularly clean. Cold door is a Chinese character, pinyin méntíngl nglu, which means it is very cold and there are few guests.

From Bai Juyi's Pipa Trip: until her brother went to war, and then her aunt died, night after night, her beauty gradually disappeared. Lengma was at the door, so at last she gave her wife to a businessman.

Usage: subject-predicate type; As predicate and attribute; Used in homes, businesses and service places.