‘Wowser’ is a delightful word with an interesting background, though its ultimate origin is unknown.
The word first appeared in print in 1899, in the Australian journal Truth, and was instantly popular in Australia. It rapidly spread to New Zealand, where it remains in use, and then eventually arrived in England, possibly brought by the Australian troops who served there during World War I.
The American writer and editor H. L. Mencken liked “wowser” and attempted to introduce it to the United States. He used the word frequently in American Mercury, the literary magazine he edited.
Despite Mencken’s efforts, however, the term never became particularly popular in American English; it is used occasionally, but it never truly caught on.