lavabo.
1. Basin used by a priest for ritual washing at the Offertory during Mass.

2. Stone trough or lavatory used for washing in medieval monasteries: some were free-standing (e.g. at Mellifont Abbey (c.1200), Co. Louth), and some were inside the cloister-wall (e.g. Gloucester Cathedral C14).

3. Washstand
.

lavabo (in the Roman Catholic Church) a towel or basin used for the ritual washing of the celebrant's hands at the offertory of the Mass. The word comes (in the mid 18th century) from Latin, literally ‘I will wash’, in Lavabo inter innocentes manus meas ‘I will wash my hands in innocence’ (Psalm 26:6), which was recited at the washing of hands in the Roman rite.

lavabo (eccl.) ritual washing of the celebrant's hands. XIX. — L., 1st pers. sg. fut. ind. of lavāre wash, LAVE; first word of Ps. 26: 6 ‘Lavabo inter innocentes manus meas’ (I will wash my hands among the innocents), the recital of which accompanies the ceremony.