From Bulbapedia, the community-driven Pokémon encyclopedia.
The Used TM is an item that exists in Pokémon Mystery Dungeon and its sequels. It is, as suggested, a TM that has been used, and it is now virtually useless. However, it can be restored using the move Recycle.
The player can check which TM a Used TM use to be by checking the item's info. In Pokémon Mystery Dungeon 2, a Used TM for Recycle is the only one that cannot be restored.
The player is unable to store a used TM. This is likely so that the game will not forget which TM it used to be.