'arrha' originates from Medieval Latin, specifically the word 'arrha', which was borrowed from Ancient Greek 'ἀρρα' where it meant 'earnest' or 'pledge'.

'arrha' entered Late/Medieval Latin from Ancient Greek 'ἀρρα', was used in legal and ecclesiastical Latin, and was borrowed into Middle English as 'arrha'/'arrhae', preserving its sense in formal/legal contexts.

Initially it meant 'earnest' or 'pledge', and over time it retained that basic sense but became a rare or chiefly legal/archaic term in modern English meaning 'earnest money' or 'deposit'.