What does 'tumtum' mean in Jabberwocky?

'Tumtum' describes the tree by which the boy stood in the poem Jabberwocky. The first letter is capialised, so it is probably meant to be a type of tree. However, Lewis Carroll did not ever say what he meant by this Word so, as readers of the poem, we are free to decide what it means for ourselves.

In his book The Annotated Alice, Martin Gardner says, "Tum-tum was a common colloquialism in Carroll's day, referring to the sound of a stringed instrument, especially when monotonously strummed."