Thirsty means feeling very thirsty, which is when your body needs more water. It's like having a big empty cup that really needs to be filled.
When people say they're thirsty, it usually means they need something to drink right away. You might hear someone ask for a glass of water because they're thirsty after playing outside or working hard in the sun. Maybe you'll see them drinking a whole bottle of juice because they've been thirsty all day. If someone's feeling tired and grumpy too, being thirsty is probably making things worse.
Needing to drink.
Example: After all that work I am really thirsty.
