Slang term used by the British Army for washing. Originates from the Indian term for washer man or woman - so probably adopted by the forces when we served in India! We used to have a Dhobi Wallah to do our Dhobi in the old days. To perform said Dhobi one needs 'Dhobi Dust' or 'Dhobi Grit'.
See also Submariner's Dhobi