the Lost.dir folder is apparently a recurrent trap for Android users of all kinds, including those running Amazon’s version, the Bellini 5.X Fire OS. It simply warehouses files recovered during system reboots, download interruptions, power outages, copying problems, and sundry other reasons. And it doesn’t delete them.

Yes, you’ll lose your recovered files if you delete the Lost folder or its contents, so you may want to back the folder up elsewhere. But normal operation doesn’t suffer. The Lost folder itself will simply reappear in due course, and in due course, you can later hose it out again. But anyway, in just a few months of usage, my Lost folder had already bloated that big. Lose the Lost. You know it makes sense.