From Latin piare = pium reddere, 'cleanse', 'expiate' (Plaut. Men. 517; Varro, Ling. 6,30), later also 'reconcile' (Plaut. Asin. 506; Verg. Aen. 6,379). Piaculum denotes on the one hand the action leading to violation of the pax deorum and requiring expiation (Plaut. Truc. 223; Varro, Ling. 629) and on the other hand the ritual act of expiation for such an offence or the sacrificial animal used for this purpose (Cato Agr. 139).
Since correct observation of instructions, acts and rules was an essential part of the exercise of Roman cults, there was a g…