"What ho!" - this strange form of greeting is used all the time by Bertie Wooster, a character of well-known "Jeeves and Wooster" stories by P. G. Wodehouse.
Bertie Wooster: Oh, what ho, Sir Watkyn!
Sir Watkyn Bassett: Kindly do not address me in that familiar way, Wooster. I happen to know that once again you've yielded to the awful temptation to steal a policeman's helmet! .....
Bertie Wooster: Aunt Dahlia! What ho, old blood relation!
Aunt Dahlia: [affectionately] Hello, Bertie, revolting young blot.
What does "ho" mean in such expressions? Is it a word reduction or an idiom?