Groggy is how you feel when you are up but not yet yourself: weak, a little unsteady, and thinking in fog. Illness can do it, and so can being very tired, waking too fast, medicine at the dentist, a bump on the head, or drink. You can still walk and talk. You just would not want to sit a test yet. It is informal and kind. You can tell a friend you are groggy, and a doctor may use it because patients already know it. It is weaker than drunk and far from unconscious. Though actually, the word started with drink. Grog was the rum mixed with water that British sailors were given; groggy first meant shaky from that. Everyday English kept the shaky, foggy feeling and mostly dropped the rum. Students often think it only means hungover. People use it just as often for the morning after too little sleep, or the afternoon after a fever.