Americans think that someone who feels “giddy” is experiencing a positive state of being, cheerful in an almost irrational or childish way. For instance, in the movie Indiana Jones and the Last Crusade, Indy’s father is described as having once been “as giddy as a schoolboy.”
But in contemporary British English, “giddy” means something negative. To a Briton’s ears, if you’re feeling giddy, you’re feeling dizzy, unbalanced, or on the verge of a headache. A good illustration of this is when the red traffic engine in the Thomas the Tank Engine TV show was once described by the voice-over narrator this way: “Poor James, feeling quite giddy, rolled off to the shed without a word.”
The alternate use of the word giddy is good to know if you’re a Yank who is admitted to a British hospital due to some misfortune. To be sure, modern British doctors use the word “dizzy” all the time, too, and doctors would ask you if you’re dizzy if they were evaluating you in the ER. But generally speaking, if Britons ask you if you’re feeling giddy, you’ll now know they’re not asking if you’re happy.