Italian, literally 'in chapel style'.

• chapel from Middle English:

  The first place to be called a chapel was named after the holy relic preserved in it, the cape of St Martin. The Latin word cappella, meaning ‘little cape’, was applied to the building itself and eventually to any holy sanctuary. Chaplain (Middle English) is a related word, which referred initially to an attendant entrusted with guarding the cape. The Latin form remains unchanged in the musical term a cappella, which means ‘sung without instrumental accompaniment’ but is literally ‘in chapel style’. See also cap