The story
Blend of dependent + hippopotamus or similar, the second element being a reference to stereotypical obesity (and the popular misconception that hippos are largely made of fat).
Etymology from Wiktionary, CC BY-SA 4.0.
dependapotamus
Today it means: A servicemember's partner (especially a SAHM) who behaves in a parasitic manner, especially by taking advantage of their partner's status to demand beneficial treatment. Full definition →