Definition: an ancient name for China
It may stem from a Turkish word to describe an ancient ruling dynasty. Marco Polo used the word quite often when describing his travels.
Why I Looked It Up
I had always been familiar with the airline Cathay Pacific. (Their elite mileage program was once called The Macro Polo Club.)
Then, in The Mystery of Dr. Fu-Manchu, it’s mentioned a few times.
For example, when the titular character is deciding how the two protagonists should be killed:
“O God of Cathay,” he cried, “by what death shall these die – these miserable ones who would bind thine empire, which is boundless!”