Suitcases of the mind? What is a "portmanteau"

The usage originates in Lewis Carroll's book Through the Looking-Glass (1871), in which Humpty Dumpty explains to Alice the coinage of the unusual words in Jabberwocky^[3]:

• "‘slithy’ means ‘lithe and slimy’... You see it's like a ? —there are two meanings packed up into one word"
• "‘Mimsy’ is ‘flimsy and miserable’ (there's another portmanteau ... for you)".

Carroll uses the word again when discussing lexical selection:

Humpty Dumpty's theory, of two meanings packed into one word like a portmanteau, seems to me the right explanation for all. For instance, take the two words "fuming" and "furious." Make up your mind that you will say both words ... you will say "frumious."^[3].

Carroll suggests here a double metaphor: the original meaning of the word 'portmanteau' is a form of suitcase (which supports the idea that meanings can be 'packed' into it), and the word 'portmanteau' is itself a 'portmanteau word', deriving from the two French words porter (to carry) and manteau (cloak or mantle).