From Bulbapedia, the community-driven Pokémon encyclopedia.
A Skitty and a Wailord at the Day Care
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.
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.