Wuv Woo is a skull in Halo Wars and Halo Wars 2.
Instead of the normal beams Scarabs shoot, all Scarabs now shoot a rainbow colored beam with hearts coming off enemy targets getting hit. In Halo Wars 2, this applies to Locusts as as well.
The Wuv Woo Skull viewed in the Halo Wars skull menu.
List of Skulls in the Halo series