The phrase often implies taking quick advantage of a situation, sometimes seen as smart but potentially opportunistic. When the celebrity scandal erupted, the magazine cashed in with a special issue.
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Possibly Negative
It can carry a slightly negative connotation if someone is seen to benefit unethically from a situation. Some say that he cashed in on his friend's misfortune.
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Colloquial Usage
The phrase is considered informal, more often used in everyday, conversational language. She intends to cash in on her cooking skills by starting a YouTube channel.