'lavabo' originates from Latin, specifically the word 'lavābo', where the root 'lav-' (from 'lavare') meant 'to wash' and the ending '-bo' was the first-person future ending ('I will').

'lavabo' began as the Latin future form meaning 'I will wash' (appearing in liturgical Psalm texts), was adopted into Church Latin and medieval ecclesiastical use to refer to the action and the basin used for washing, and then entered English as 'lavabo' (via ecclesiastical/Latin usage).

Initially it was a verb form meaning 'I will wash (my hands)'; over time the term shifted to name the basin used for washing and the associated ceremonial act.