Not quite there. A spacey person is awake and present in the room but only half-connected to it — slow to answer, staring, losing the thread, forgetting what they came in for. It is informal, gentle, and usually not an insult; you say it about yourself when you have slept badly ('sorry, I'm really spacey today'), or about a friend kindly. It can be a passing state (tired, ill, on medicine, just woken up) or a description of what someone is generally like — a dreamy, unfocused sort of person. There is a second use, for music, film or art: spacey sounds are floating, echoing, dreamlike, the kind of thing you would imagine drifting through space. That use is a compliment. Be aware of one shadow on the word: because being 'spaced out' is also how people describe the effect of some drugs, calling an adult spacey can sound like a hint about that if the situation invites it — among friends it is harmless, but I would not use it about a colleague in front of their boss. The spelling wobbles: 'spacey' and 'spacy' are both used, with 'spacey' more common.