A strong contender for '2-letter Word of the Year 2007'?
From the OED: 'A river, running water. Still in use in Lanc.; in the fen-country applied to the canals for drainage, in which sense it is usually spelt eau [as if a. F. eau water].'
Unlike many of the two-letter oddities with which internet scrabblers have been confounding me of late, this one I could see myself getting some use out of.