The word "carat" in English is the unit of weight of diamonds, and one carat is equal to 200 milligrams. In old Shanghai, diamonds over 3 carats were very scarce and expensive. So whenever you can see such a "good thing" in a jewelry store, the old master in the store will give a thumbs-up and say, "This old Keller".
Later, this word was specially used to describe some people who accepted western culture earlier in old Shanghai. These people live a westernized life and receive a good education. They usually find a decent job in a foreign company. They earn a lot, so they dress well and have more pursuit of leisure life and culture.
Extended data:
Mu Xin made a more incisive description of the attention of Lao Keller:
Shanghainese can tell at a glance which way your suit was made, and even decide which store made it. When the servant hangs your coat, he habitually glances at the brand name. Foreign high-end clothing stores embroider your Chinese and English names with silk thread and sew them on the upper edge of the inner chest pocket. Shirts and handkerchiefs are also specially embroidered. The shirt is ironed and worn now, which is crisp and lively enough.
After the tie is taken off, it will be shaped with a splint. Clothes hangers and shoe last are customized according to the actual situation. Take a shoehorn before putting on your shoes. Regardless of stockings and socks, be sure to hang them with elastic bands. If the socks are seen wrinkled, "this man is so worthless." It's ridiculous to wear black leather shoes in summer. They are all white leather shoes. Yellow skin and color-spring and autumn, black skin and suede-winter.
Baidu Encyclopedia-Old Keller