Impiety is a disrespect for the sacred. For example, visitors are advised not to wear shorts or tank tops when touring certain churches and cathedrals in Europe, because doing so is viewed as impiety by those who worship there.
English offers many options to describe disrespect. Impudence, insolence, sass, and irreverence are a few choices. Impiety sets itself apart from these in that it describes a lack of respect for a deity, like a god or God, or for worship itself. Piety is devotion or reverence, so impiety is a lack of devotion or reverence — such as making fun of other people's religious beliefs.