What is the meaning of "back smb up"?
It is rather hard to prove that a man is not guilty and he says this phrase: Det. Wade : So you can only assert that he said it. You can’t prove it.
Jane : No, I can’t, but the gun he’s carrying certainly BACKS ME UP, doesn’t it?

Question

What is the meaning of "back smb up"?
It is rather hard to prove that a man is not guilty and he says this phrase:  Det. Wade : So you can only assert that he said it. You can’t prove it.
Jane : No, I can’t, but the gun he’s carrying certainly BACKS ME UP, doesn’t it?

Answer

Supports his statement he made.

Answer

Back up somebody = "To have one's back is to protect one from unanticipated matters, while one is attending to a particular function."

Answer

in this context, they are saying that the gun adds weight to the argument that the man is guilty.

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