It's a joke . .
The middle finger is often used to make an obscene gesture, so it is held up to give a message to another person - especially when one person is mad at another.
Because the finger got cut off, he will not be able to "give the finger" or "flip the bird" to another person.
As a result, he is forced to be silent or unable to "speak" (give) the message.
Like I said, it is a joke.