'"Don't let the cat out of the bag"
This means 'don't tell anyone our little secret.'
Example: "Listen, Joe, I'm starting a consulting company, and you've got a great government job. So, how about you give me some of those lucrative government contracts and don't let the cat out of the bag?"
"I'm feeling under the weather"
This means 'I'm sick.' (But I can't really think of a good thing I'm sick from exactly)
Example: "I would love to go to that thing the company is doing for the environment today, Joe, but I'm a little under the weather. Yeah, I think I have Idongivaratsass."
"Once in a blue moon"
This means that it happens rarely
Example: "When you're praying, do you ever think God that you're not French? Yeah, once in a blue moon, but I don't really think about other countries that much."
"She twisted my arm"
This means she really convinced you to do something and wouldn't take "no" for an answer. Example: "Trust me, I usually wouldn't be caught dead at a necro-orgy, but she twisted my arm."
Can you think of other purely American idioms? What's your country's equivalent?