Make sure that the Web Store Out Of Stock Items behavior at Setup > Site Builder > Set Up Web Site > Shopping tab and the item record Out Of Stock Behavior at the Web Store tab is set to Allow back orders but display out-of-stock message.
Also check if the Description field in the Cart tab of Setup Website is enabled:
1. Navigate to Setup > Site Builder > Set Up Web Site > Cart tab.
2. There are 2 Descriptions that you can show on the cart table (Description and Detailed Description).
3. The one that has the out-of-stock message is the Description so make sure that it is enabled.