Combination of the names 'Zabuza' and 'Haku', a canon-to-some/fanon-to-others yaoi couple from the anime Naruto. Zabuza took Haku in, and raised him to be the ultimate
ninja and the ultimate tool. Haku's feelings, whether you want to imply shounen-ai or not, were
made glaringly obvious by his unwavering devotion to Zabuza. After all was said and
done, though, they *both* really cherished
one another.
'ZabuHaku' implies that Zabuza is the dominant partner (as opposed to 'HakuZabu'), but both can be used interchangeably.