Whereas the prefix meta means "that transcends", "that encompasses"; then meta-giantess means "giantess of giants".

It occurs when there is a 2 giant beings interact, so that if the greater being is female, then she is a meta-giantess.

A good example is the following:

2 vs 1 demolition competition by masterofra-d8oi44v

Maddie (the girl in all-blue dress) is the meta-giantess in this case, since she interacts with other giant beings: Chibiusa (the girl with pink hair) and McKenzie (the girl in black and white plaid skirt). Illustration by MasterOfRa.