"Saint Patrick's Breastplate" is an Old Irish prayer of protection of the "lorica" type (hence "Lorica Sancti Patricii", or "The Lorica of Saint Patrick") attributed to Saint Patrick.
Its title is given as Faeth Fiada in the 11th-century Liber Hymnorum that records the text. This has been interpreted as the "Deer's Cry" by Middle Irish popular etymology, but it is more likely a term for a "spell of concealment".[1] It is also known by its incipit (repeated at the beginning of the first five sections) atomruig indiu, or "I bind unto myself today".