What's the meaning of もの in this sentence?

Question

> てっきり、兄は腰を下ろす**もの**だと思っていた。 I have no clue about what it could mean.

Answer 1

"もの" here just means "the person," which would translate to "I thought the person sitting down was [my] older brother for sure."

In other contexts, it can also mean "the thing," if we're not talking about a person.