Scammers specifically hunt for wealthy easy marks.
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usage
Almost always figurative in modern Chinese. The literal sense ('fat sheep') survives mostly in restaurant names like 小肥羊 (the famous hot pot chain), where it's a positive descriptor of mutton.
register
Colloquial. The figurative sense - 'sucker / mark' - has a slightly cynical, predatory edge: the speaker is implying someone is about to be fleeced.