Something that's unhallowed hasn't been blessed by a bishop or other religious authority. If a body is buried in "unhallowed ground," it's not a church-sanctioned cemetery or graveyard.
An object, building, or piece of land that's hallowed is considered to be holy, or officially blessed by a priest or other religious authority. The words hallowed and unhallowed are used most frequently in religious speech or writing, often Catholic. A proper Catholic burial, for example, would be in hallowed ground. Both words come from the Old English halgian, "to make holy, consecrate, or ordain," related to halig, or "holy."