I think the above is the most likely, it is also know as Arni's Haug
But there is also the following theory which I think is unlikely....
"A document dating from the reign of Edward III, dealing with an enquiry into the question of encroachments into the Forest of Wirral, treats with the occasion when Richard de Oxton was called to account for having in his possession a quarter of a rood of land in Oxton, near Arnehowe. At the period of this document, the use of the words erne, earn or arne, for eagle was quite common. How is well-known as a word for hill, so could it have been that the name for the Arno in medieval years was Eagle Hill? "
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates