Having seen the vetters' comments on an early draft of these questions, I can report that the phrase "usually used in poetry" was added at the suggestion of the vetters, who felt that this question was "very hard".
Personally, I can't see how the addition makes it any easier. I like to think I know the English language quite well, and I'd never heard of a kenning before I heard this question.
Wikipedia tells us that the word comes from Old Norse, and the device is "strongly associated with Old Norse and later Icelandic and Old English poetry."
Just the kind of thing we all read, and study, all the time.
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