Flesh is a type of Item obtained from enemies in the Towers. They are found in the Flesh tab of the Bag, which is the fourth tab from ALL. There are 5 types of Flesh, but you can only get three from Servant Beasts. When you return to the Observatory with the Flesh, you can either give it to Elena to reset the Curse Timer to a certain point or sell it to Mavda. If you take it back into the Towers, it will immediately turn into Dried-out flesh, no matter what type of Flesh it originally was. Because of this, it is impossible to put Flesh in the Trunk. Mavda will not pay you very much for Dried-out flesh.