Portmanteau is a French word, in the singular. If you want to show off your knowledge of French but write in good English, write the plural of the noun as portmanteaux; portmanteaus is also acceptable in modern English writing. (If you really want to show off your knowledge of French, write the plural with the French spelling - portemanteaux - but remember to put such a foreign word in italics.) When you use the word as an epithet (adjectivally), as in portmanteau word, the plural should not be marked.
- You may also want to see portmanteau word and a note on the etymology of words containing the element '-port-'.