I can't think of any that I don't understand because if I didn't understand something I'd look it up. However, some American words and phrases that have puzzled me at some point have been: pork (government funds), to jones (to suffer drug withdrawal), rain check (a promise for something postponed), potluck (a type of shared meal), one-oh-one (the basics of a subject, from university course numbering), to visit with (to chat to), freshman/sophomore/junior/senior (high school and university year numbering), to tailgate (a party where food and drink is served from a car), to plead the fifth (to not speak in order to avoid self-incrimination), to jaywalk (to cross a road at somewhere other than a designated crossing), layaway (installment purchasing), charley horse (a leg cramp), redeye (a long-distance overnight flight), to bus (to clear a table), candy-striper (a hospital volunteer), podunk (of a small town, implying insignificance), and no doubt many others.