Someone who's spacey acts distracted, befuddled, and maybe a bit strange. You might be a little spacey after you get your wisdom teeth removed.
When people say funny things as they're coming out of anesthesia, acting silly or out of touch with reality, they're being spacey. You might also have a spacey friend who simply tends to be a little forgetful or quirky. This informal adjective, also spelled spacy, originally meant "roomy" and then "acting or feeling as if you're in outer space," probably from 1965 American slang, spaced-out.