Hargwned comes from harpwned, which came from pwned, which came from owned. It's a long long line of Internet slang typos. Basically, Hargwned takes the place of any variation of "owned", but usually it takes the place of harpwned (harpwned usually refers to getting owned by a harpoon, but it can refer to other ways of getting owned as well).
Whilst trying to harpwn a whale, Dave accidentally Hargwned himself. What a noob.