This thread at Ask MetaFilter contains several examples of English words and phrases that sound current but actually aren’t.
“Fly” mentioned above is one of my favorites (though it’s considered dated by youngsters today — they’ll basically only use it ironically), but one not mentioned so is “crib” (also a bit dated) which has meant something like “house” or “home” since 1600. OED: “A small habitation, cabin, hovel; a narrow room; fig. a confined space. In N.Z. now esp. a small house at the seaside or at a holiday resort.”
I particularly liked this early example of the maternal insult from Shakespeare’s Titus Andronicus:
Demetrius: “Villain, what hast thou done?”
Aaron: “That which thou canst not undo.”
Chiron: “Thou hast undone our mother.”
Aaron: “Villain, I have done thy mother.”