Empty Bottle
noun
A sky bottle is a bottle that has nothing but sky in it. It's an empty bottle.
あきびん
The reading is a combination of kun'yomi and on'yomi readings. You learned this reading for 空 when you learned 空き缶. If you can remember that, you can remember this too!
空き瓶はこちらに捨ててください。
Please throw away empty bottles here.