It doesn't appear that this expression was a minced oath or something along those lines. Was it shortened from a longer phrase, or did it just enter the vernacular as is (similar to "listen up" or ...
"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 ...
How should this expression be used, and what is its origin?