A portmanteau is a word formed by combining two or more elements of other words. In linguistic terms, these words are known as blend words.
The expression fortnight is an example of a portmanteau and it derives from the Old English word féowertýne niht, which meant fourteen nights. féowertýne niht appears in English print at the start of the eleventh century, but it wasn’t until the early eighteenth century that the portmanteau fortnight became established in English print, with many spelling variations up until this point.
But let’s backtrack a little…why do we use this French expression portmanteau, anyway? Who started adding French words to our vocabulary? It was Lewis Carroll. He coined the term portmanteau when he introduced new words into the English language through his literary work, including the portmanteau chortle (chuckle+snort). In Carroll’s classic story Through the looking glass, and what Alice Found there (1871), Alice asks Humpty Dumpty what slithy and mimsy means and he explains: “Well, “slithy” means “lithe and slimy”. “Lithe” is the same as “active”. You see it’s like a portmanteau — there are two meanings packed up into one word”. In the sixteenth century, portmanteau luggage was a hinged case or bag separated into two compartments. This, itself, derives from the French words porter (to carry) and manteau (coat). Humpty Dumpty uses the word portmanteau to help Alice visualize the “packaging” of different linguistic elements to form one word that “carries” all the elements.
Other examples of portmanteaus (or, portmanteaux) include: motel (motor+hotel) and smog (smokey+fog). Modern examples include blog (web+log), intercom (internal+communication) and pixel (pictures+element).