This will probably be a bit of a longer explanation but i kinda love telling that story... Back in the days like 15-20 years ago i played table-tennis on a pretty good amateur-level (at least good enough to have some spending money while going to school / starting university)...
Sporting 120-130kg at 180cm i was the opposite of an athlete - pretty hilarious, always looking like getting a heart attack few minutes into the games... Making up for that with just being to stubborn to give up AND being loud, very loud...
A teammate caught sumo-wrestling on tv and thought Yokozuna was a good new nickname for me, so i kinda had to deal with it.
I did hate it so much that at some point i got supermad and was yelling at the guys, kinda ready to start a fight:
"fucking idiots! this is not funny anymore... you assholes, so stupid that you cant come up with something else!?" - not wasting any time, one of them answered: "Well, Takanohana would be too long"... followed by a few seconds of awkward silence... and suddenly everyone bursted out laughing - me included... the insta-delivery just was so funny, made it impossible to stay angry...
Having 1 or 2 beers after the game transformed into getting falling-down drunk together and "Takanohana" got shortened to "Takka"... God knows why, but somehow that nick really stuck & spread quickly - more or less replacing my real name..
These days my mum is kinda the only one calling me by my first name... Even at work everyone goes with my nickname, which is always funny when a new intern says my real name and only gets "Who?" and some weird lookes for that. ;-)
tl;dr: fat guy getting nicknamed after a sumo-wrestler... nickname stuck even after losing 40kilos+
edit: you know those suggestions you get on some sites when trying to sign-up with a name thats already taken? thats how the "one" got added by aol ages ago