Their definitions seem to blur for me, as well as compound words in general. I'm thinking it might be a "square is a rectangle but a rectangle is not a square" concept but I'm not certain.
Examples are also greatly appreciated.
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.Sign up to join this community
The words portmanteau and kenning come from different contexts and refer to different kinds of compounding. Kenning was first used in English by 19th century scholars trying to describe a feature of Old Norse and Old English poems: the use of periphrastic compound expressions instead of a simple description of a thing (OED). Here are a few examples that show the compounding at work (Wikipedia):
Kennings often involve some kind of creative analogy, like imagining a sea as a road for whales, or the sun as a candle in the sky. Introductory literature courses often introduce kennings as a feature of Old English; if someone is talking about English in general, they are more likely to talk about a compound of some kind, like the noun compound "seat belt".
Portmanteau in English has referred to a specific kind of bag since the 16th century (OED). In the 19th century, Lewis Carroll adapted the term to refer to a word blending representative sounds from two or more words (CJR):
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.
So portmanteau don't just put two words together into a compound, but delete parts of the word to more fully blend them. So it's not smoke-fog but smog; it's not brother-romance but bromance.