Which part is a metaphor in the line from Thank You Ma'am by Langston Hughes The large woman simply turned around and kicked him right square in his blue jean sitter?

Remember, a metaphor can only occur within the context of the statement. So if I said, "She leaned on her elbow as she slept", there is no metaphor, just a subject, verb, and object phrase.

So ignore standard parts of speech in "The large woman simply turned around and kicked him right square in his blue-jeaned sitter." That means these are ignored: woman, turned / kicked, and all adjectives and adverbs. We're left basically with "blue-jeaned sitter". But what does that mean? Think about it. What would be inside blue jeans that could be called a "sitter"? The metaphor is made between buttocks as a "sitter".

The metaphor could work too if it said " his blue-jeaned padding" but that is not as vivid as "sitter". Sitter also keeps with the overall tone and time (e.g. the period) of the writing.