Here's why:
* The image: Imagine a cat, known for its stealthy hunting skills, proudly strutting around after successfully catching its prey (the canary). The cat's puffed-up chest and triumphant demeanor create a visual representation of smugness.
* The meaning: The idiom signifies that someone is displaying obvious signs of pride or satisfaction, often because they've achieved something that others haven't.
Examples:
* "After winning the lottery, he walked around looking like a cat that swallowed the canary."
* "She got the promotion over all the other candidates, so she was practically radiating with a 'cat that swallowed the canary' attitude."
Essentially, the phrase paints a picture of someone who is clearly enjoying their perceived success and is letting everyone know about it.