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
No, 'tumtum' is not a portmanteau. A portmanteau is a blend of two or more words to create a new word with combined meanings. 'Tumtum' is a single word that does not combine or blend multiple words.
skookum tumtum
A neologism for a portmanteau created by incorrectly combining a malapropism with a neologism. It is itself a portmanteau of 'malapropism' and 'portmanteau'.A malamanteau is a neologism for a portmanteau created by incorrectly combining a malapropism with a neologism. It is itself a portmanteau.A word defined to infuriate Wikipedia editors
yes
The Tumtum tree is the tree by which the boy rested in the poem Jabberwocky, which appeared in the book Through the Looking Glass, the sequel to Alice's Adventures in Wonderland.He took his vorpal sword in hand:Long time the manxome foe he sought --So rested he by the Tumtum tree,And stood awhile in thought.Carroll offered definitions for many of the words in the poem Jabberwocky, but never described what he meant by Tumtum tree. It is arguable that the nonsense words don't really have or need proper definitions, and it is the sounds of the words, the feelings they invoke and the ideas they suggest which is what is important, therefore, all that matters is what 'Tumtum tree' means to you, as a reader.However, in his Annotated Alice, Martin Gardner says that "Tum-tum was a common colloquialism in Carroll's day, referring to the sound of a stringed instrument, especially when monotonously strummed," so perhaps this is the inspiration behind the term.
Reptavian.
Mammavian.
A gorillaphant.
An elephanttaur.
Motel
Insectomammal.
Motel
A giraffephant.