The cat that swallowed the canary

What does the phrase "the cat that swallowed the canary" mean exactly?

I haven't heard this one before, but I suspect it is meant to describe someone who is very pleased and smug because he's got what he wanted. I think it might be similar to "the cat that got the cream". It would help if you could use the expression in context.

You are right on the money! It is a commonly used expression in these parts.

look like the cat that ate the canary
Also, look like the cat that swallowed the canary.
Appear smug and self-satisfied.
For example, "After she hit her third winning shot, Jeannie looked like the cat that ate the canary.
[Second half of 1800s]
From The American Heritage Dictionary of Idioms by Christine Ammer

QUOTATION: That's what the cat said to the canary when he swallowed him: "You'll be all right."
ATTRIBUTION: Alvah Bessie, Ranald MacDougall, and Lester Cole. Raoul Walsh. Mr. Williams (Henry Hull), Objective Burma, having been assured he'll be "all right" on his first parachute jump .
From _The Columbia World of Quotations_