inside

This clearly looks like an upside down box. Doesn't it?

Inside a person, there's a lot of meat.

This term is more old-fashioned and formal, and it's often used by older men when referring to their own wives, particularly in formal or public settings. It carries a more humble and traditional connotation.

Note how the kanji in this word correspond to traditional gender roles. Therefore, younger people tend to prefer using the word 妻つま.