From Bulbapedia, the community-driven Pokémon encyclopedia.
HSOWA is an acronym that stands for Hot Skitty On Wailord Action. It is derived from the fact that Skitty and Wailord can breed in the games despite the massive size difference. It has reached cult status on some message boards.
The term originates from GameFAQs where, when this topic came up, the user 'Endgame' reportedly said "HOT SKITTY ON WAILORD ACTION!". The name then stuck.
Though Skitty is quite small, another Pokémon in the same egg group as both Skitty and Wailord, Diglett, is even smaller, at 0'8". The reason that there is no HDOWA is possibly that Diglett and Wailord are not found in the wild in the same region, thereby only being together in the National Pokédex listing, not one of the regional Pokédex listings. Plus, since only its head is visible, Diglett's true size is unknown. This can get even stranger with Seedot who is not only smaller than Skitty, but also based on a plant, and yet can still breed with Wailord. It also is in the Hoenn Pokédex, and is found in the wild in the same region. Also, Eevee, another cat-like Pokémon, and Rattata, a rat Pokémon, who are both shorter than Seedot and Skitty, but taller than Diglett, can also breed with Wailord. Apparently, however, the Skitty pairing was more famous.
A fanmade sprite depicting the crossbred result
The reason why Wailord is included in the Field Egg Group, the same group Skitty is in, in the first place is likely because whales are mammals, like most other Pokémon in the Field group.