Dropped by
it does nothing
No recipe, no fuel, no use of any kind. It exists to be a bad gift, and the game is quite committed to the bit.
the joke
Presents drop it because of the old threat about badly behaved children waking up to a lump of coal instead of a gift. Jungle Mimics hand it out far more freely, which is its own punchline given what a mimic normally guards.
the one item that will not stack
Everything else in the game piles into a single slot. Coal does not, so a handful of them is a handful of inventory gone. That is deliberate rather than an oversight, and it is the closest this item comes to having a mechanic.