Definitions

1. adjective

The most fat or obese; the most having a tubby or stout appearance

“She was known as the tubbiest person in her high school class.”

2. superlative form of ‘tubby’

Comparing three or more things, the one that is most fat or obese; the one that has the most tubby or stout appearance

“Of all the friends at the party, John was the tubbiest.”