Australian slang used to exaggerate size and imply a negative conotation. Usually considered a derogatory term and is most often used in relation to someone who is morbidly obese
To be grammatically correct the term may either be:
placed immediately before a size related adjective e.g. That port-a-loo is vellis big; or
used as an adjective e.g. He's so fat
, he's vellis.
Terms is possibly derived from large beast described in ancient Greek