Is 'tumtum' a portmanteau

Lewis Carroll never offered an explanation for the Word 'tumtum' but, owing to its structure, it is unlikely to be a portmanteau.

Other portmanteau Words combine two different Words to make a new one, for example, 'miserable' and 'flimsy' make 'mimsy'. If 'tumtum' were a portmanteau, it would have to be a combination of the Word 'tum' and 'tum' again, or two different Words in which 'tum' is the first syllable of one and the second or last syllable of the other.

Of course, while unlikely, this is not impossible, and there are many Words which might be combined to create 'tumtum'

For example

Words with 'tum' at the beginning

• tumble
• tumbril
• tummy
• tump
• tumult

Words with 'tum' at the end

• arboretum
• dictum
• erratum
• factotum
• sanctum

However, the rhythmic, repetitve nature of the Word suggests that it was more likely to have been chosen simply for its sound.

In his 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.