Umaa

Umaa may sound like an injoke, but in fact came from Mutt's worldbuilding: it is a serious species, and the meme came later.

An umaa (plural: umaa) is a cowlike creature. They appear in Green's world of origin and are used by grundraak as beasts of burden and for food.

The word 'umaa' is onomatopoeic: that is, the beasts are named umaa because they say "umaa".

Profusionites often bellow "Umaaaa!" in greeting, celebration, confusion or triumph. This dates back to an unfinished early version of Green's history that Mutt was writing, in which one character excitedly exclaimed it.