Are there rules for constructing portmanteaux?

Lewis Carroll popularized the use and creation of (what may be considered to be) a special form of compound or conjoined words. I propose that these are different than other compound words (e.g., per the comments, “grimdark” from grim+dark)

Since the concept of a portmanteau is by nature poetic and inventive, I think it is most productive to focus on the notion of disallowed constructions.

For instance, I think that a portmanteau:

• cannot use a classical prefix or suffix as one half of the constructed word.

  (unbor, as un + bor-rowed )

• cannot have parts that are, themselves, whole words

  (e.g. understanding)

Either way, do rules exist in the construction of portmanteaux?

Do all portmanteaux by nature have a light-hearted tone? Must they “sound good”? Or does their “lightness” arise from the wit used to create them?

(edit: incorporated comments)

Depending on how loosely one defines either, sniglets are sometimes considered portmanteaux. So given the sense of portmanteau as a particular sort of neologism, if you’re looking for short and simple rules for such coinages, you might also look into sniglets.

Although Lewis Carroll’s works of whimsey are oft cited as the origin of our modern portmanteaux, and certainly of the word portmanteau to describe these, many of the words he invented don’t quite fit the somewhat narrow sense now given for English portmanteau words.

His words really are much cleverer than simplistic coinages like brunch, infotainment, or ginormous, clearly no more inventive than simple splicing. So it is somewhat of a disservice to Mr Dodgson to describe his delightful inventions as mere portmanteaux, at least as that word tends to be used today.

If you’re trying to derive or infer rules for the construction of such things, you could do far worse than look at translations of Carroll’s whimsical poem, “Jabberwocky”. Contrast the English with each of several Romance translations:

Twas brilig, and the slithy toves
Did gyre and gimble in the wabe;
All mimsy were the borogoves,
And the mome raths outgrabe.

Il brilgue: les tôves lubricilleux
Se gyrent en vrillant dans le guave,
Enmîmés sont les gougebosqueux,
Et le mômerade horsgrave.

Era la asarvesperia y los flexilimosos toves
giroscopiaban taledrando en el vade;
debilmíseros estaban los borogoves;
bramatchisilban los verdilechos parde.

Era brillosto, e gli alacridi tossi
succhiellavano scabbi nel pantúle:
Méstili eran tutti i paparossi,
e strombavan musando i tartarocchi.

Hora aderat briligi. Nunc et Slythia Tova
Plurima gyrabant gymbolitare vabo;
Et Borogovorum mimzebant undique formae,
Momiferique omnes exgrabure Rathi.

Douglas Hofstadter presents French and German versions in Gödel, Escher, Bach, with some discussion of what goes into making such choices. Martin Gardner in The Illustrated Alice gives a detailed analysis of the French and German translations. Both those might help you to come up with “rules” for these things.

Basically, in order to qualify as a portmanteau the term has to blend rather than simply splicing together the root terms. So grimdark doesn't qualify, but smog does. It's possible for a root term to appear in its entirety as long as the other doesn't, as in Eurasia.

I would also say that in order to be a good portmanteau, the "join" part of the word shouldn't be an awkward morphological formation for its language (e.g. trying to portmanteau crack and addict into crackdict). Extra points are awarded for joining on a common letter or phoneme, as with spork.

There is no particularly requirement of lightheartedness; the aforementioned Eurasia is perfectly serious in tone, for example.