grammar - What's the meaning of もの in this sentence?

てっきり、兄{あに}は腰{こし}を下{お}ろすものだと思{おも}っていた。

I have no clue about what it could mean.

Answer

「Phrase/Mini-Sentence + ものだと思っていた」

is probably better if you remembered it as a fixed expression meaning:

"I just took it for granted that (Phrase/Mini-Sentence)."

"I never had the slightest doubt that (Phrase/Mini-Sentence)."

It would be practically impossible to translate the 「もの」 all by itself here, but it is being used to refer to an "assumed fact".

「てっきり、兄{あに}は腰{こし}を下{お}ろすものだと思{おも}っていた。」

therefore means:

"Without thinking, I simply had assumed that my older brother would sit down."