I think this is a British idiom.
The American version would be, "The cat that killed the canary."
I was about to say this to a female friend, intended as a "well done" sort of compliment, specifically on something to do with her relationship, when my internal filter suddenly held back. Given there exist innuendo definitions of "cat" and "cream", and that she's American, not British, would the phrase have mis-communicated some innuendo?
Best Answer
In my 51 years of being British, and 50 years of speaking British English, I can safely say I have never come across any innuendo associated with the phrase "The cat that got the cream".
My understanding of the phrase would be that if two or more cats were given a saucer of milk from the same bottle, the one that got the cream (that floats at the top of the bottle) would be smug, because the cream is richer and tastier than the milk that follows.
For additional info, I am part of a family that thrives on innuendo, so I would consider myself a reliable source on this subject. Now, if the phrase was "The pussy that got the cream", I might well snigger.