A kenning (Old Norse kenning, plural kenningar) is a stylistic device that was commonly used in Old Norse poetry. It’s a form of periphrasis (referring to something indirectly) that uses images from a body of traditional lore to designate something rather than calling it by its everyday name.
A simple example would be “man of rings,” a kenning for “king.” Kings were men who gave out rings and other finery to their followers, so it was fitting to call a king a “man of rings.”
However, kennings were typically more complex and enigmatic than that. Here’s an example of an extremely complicated set of them:
The splendid hater of the fire of the sea defends the beloved of the enemy of the wolf; ships’ prows are set before the steep brows of Mim’s friend’s wife. The noble mighty-ruler knows how to hold the serpent’s attacker’s mother. You who torment necklaces, enjoy the troll-wife’s enemy’s mother until old age.
What the hell could all of that possibly mean?
Norse mythology and later Germanic folklore often associate gold with bodies of water, so “fire of the sea” is a kenning for gold. The “splendid hater of [gold]” refers to a king or other ruler, who gives gold to his followers so generously that it almost seems like he’s trying to get rid of it. The “enemy of the wolf” is the god Odin, who fights the wolf Fenrir at Ragnarok. Odin’s “beloved” here is the earth-goddess Jord, who was sometimes said to be Odin’s wife. “Mim’s friend” is once again Odin, and his “wife” is once again Jord. Jord’s “brows” probably refer to sea cliffs or mountains close to the water. The “serpent’s attacker” is the god Thor, who fights the serpent Jormungand a couple of times in Norse mythology. Thor is the son of Odin and Jord, so “the serpent’s attacker’s mother” refers to Jord. “You who torment necklaces” is another reference to a ruler, who “torments” necklaces by giving them away left and right. The “troll-wife’s enemy” is once again Thor, who frequently fights with “trolls” (that is, giants). His mother, as we’ve already noted, is Jord, the earth.
So, when the passage is “decoded,” it reads: “The king defends the land; ships’ prows are set before the sea cliffs. The noble king knows how to hold the land. King, enjoy the land until old age.” While easier to understand, that doesn’t seem nearly as rousing, does it?
In addition to using kennings to heighten the emotional effect of their poems, Norse poets evidently took great delight in phrasing their poems in such a way that they could only be fully understood by those who were as steeped in traditional lore as they were. It was a way of showing off their skills. Kennings were also mental games, allowing the audience to take delight in deciphering the hidden meaning.
Kennings are important for us today not just as a stylistic curiosity, but as a source of information about Norse mythology and religion. Many of the myths that have come down to us were only recorded in late, problematic sources such as the Eddas and the sagas. They can’t be taken at face value as preserving authentic lore from the Viking Age. But sometimes kennings in Viking Age poetry can corroborate elements of those myths, allowing us to see that, for example, the idea of a fight between Odin and “the wolf” Fenrir existed during the Viking Age, and wasn’t just invented by later, Christianized writers.
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 Abram, Christopher. 2011. Myths of the Pagan North: The Gods of the Norsemen. p. 13-14.
 Ibid. p. 14.
 Snorri Sturluson. The Prose Edda. Translated by Anthony Faulkes. p. 168.
 Abram, Christopher. 2011. Myths of the Pagan North: The Gods of the Norsemen. p. 14-15.