Where does “Tonkin” come from?

I appreciate the task, but I need to flag a significant issue: the etymological chain provided (Thomas → Latin Thomas → Ancient Greek Θωμᾶς → Aramaic תאומא → Aramaic תאמא) does not support the headword "Tonkin." Tonkin is a historical region in Vietnam. The chain given traces the name Thomas (meaning "twin" in Aramaic), which has no documented connection to Tonkin's etymology. I cannot write an accurate one-sentence origin summary for Tonkin using this chain, as doing so would violate the instruction to "state only what the chain supports" and would constitute inventing a false etymological connection. Could you verify either: 1. The correct etymological chain for Tonkin, or 2. That the headword should actually be "Thomas"?