make a killing
idiom

- to earn a lot of money in a short time and with little effort

Cambridge Dictionary

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WORD ORIGIN

It's difficult to pinpoint exactly where this phrase came from, but most sources state that it originated in the U.S. in the 19th century. Cassell's Dictionary defines making a killing as: To make a profit by gambling, whether at the races, on the stock market, in a casino, etc.

Today, the phrase is not limited to making money by gambling. It can be applied to any situation where a large profit is made, usually quickly and without much effort. Kill has been used for some time in the sense of doing something to a great or overwhelming degree.

In sports for instance, one often hears the phrase "they killed us", meaning that the margin of defeat was large. Or if something or someone is extremely funny, one might say "He killed me with that joke" or "That movie really killed me."

So taking the word kill in the context of being extreme, a "killing" becomes a metaphor for not only making money, but making a great deal of money. There are several other ways to express making a lot of money:

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SYNONYMS

break the bank, cash in on, clean up, hit the mark, make it big, rack up, rake in, score, strike it rich

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Practice OWAD in a conversation:

"They MADE A KILLING with the sale of their London house."