ken v.

Old English 1st and 3rd person singular present indicative of cunnan "to know," less commonly as an auxiliary, "have power to, to be able," (also "have carnal knowledge"), from Proto-Germanic *kunnjanan "be mentally able, have learned" (source also of Old Norse kenna "become acquainted, try," Old Frisian kanna "recognize, admit, know," German kennen "know," Middle Dutch kennen "know," Gothic kannjan "make known"), from PIE root *gno- "to know."

It holds now only the third sense of "know," that of "know how to do something" (as opposed to "know as a fact" and "be acquainted with" something or someone). Also used in the sense of may, denoting mere permission. An Old English preterite-present verb, its original past participle, couth, survived only in negation (see uncouth), but compare could. The present participle has spun off with a deflected sense as cunning.

"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)).