HSOWA - Short for the phrase 'Hot Skitty on Wailord Action'. This phrase originated from a topic on the GameFAQS Pokemon Ruby/Sapphire boards when a user noticed that Skitty, a tiny kitten only six inches tall, could mate with Wailord, a HUGE whale, 47 feet tall. Then the user, Endgame, ended up saying 'HOT SKITTY ON WAILORD ACTION!'
And it started from there. It's been shortened to HSOWA ever since.