This article's content is accurate to the most recent public game version, 0.4.8d.
"A broken bottle coated in something yucky. Better not touch it."
Garbage? is a type of consumable item whose sole function is 
poisoning a party member. It mainly acts as part of a puzzle (and time waster) on the first floor of the Fey Mansion, which is also the sole place where it can be found in up to 10 chests at random. The best course of action is to just sell these to get them out of the inventory.