"periphrastic expression in early Germanic poetry" (such as swan-road for "sea," sky-candle for "sun"), 1871, a modern learned word from Old Norse kenning in a special sense "poetical periphrasis or descriptive name" (it also meant "teaching, doctrine; preaching; mark of recognition"), from kenna "to know, to recognize, to feel or perceive; to call, to name (in a formal poetic metaphor)," from PIE root *gno- "to know."
In the whole poem of Beowulf there are scarcely half a dozen of them [similes], and these of the simplest character, such as comparing a ship to a bird. Indeed, such a simple comparison as this is almost equivalent to the more usual "kenning" (as it is called in Icelandic), such as "brimfugol," where, instead of comparing the ship to a bird, the poet simply calls it a sea-bird, preferring the direct assertion to the indirect comparison. [Henry Sweet, "Sketches of the History of Anglo-Saxon Poetry," London, 1871]
Cognate Old English cenning is attested as "procreation; declaration in court" (and see kenning (n.2)).