Eerie means spooky, creepy or suggestively supernatural. If it's eerie, it's sure to make the hair on the back of your neck stand up.
Back in the 1300s when eerie first came on the scene, it meant "fearful or timid." It took a good 500 years or so before it morphed into the adjective we know today, which now means "causing fear because of strangeness." And the strangeness is key: Something that's eerie isn't just scary. It's mysterious, ghostly, and gives you the creeps. Like dark old castles, misty graveyards and creaky sounds in the middle of the night.