Knife shows the same silent k as knee and knight, and for the same reason — the cluster was pronounced until about 1600 and the spelling was fixed before it went. But knife has a second story: it is probably a Norse borrowing, from Old Norse knifr, taken into late Old English as cnif.
That makes it one of the many blunt, everyday words the Danish settlement left in English. Norse gave English take, want, die, law, window, husband, egg, skin and sky — vocabulary so ordinary that nobody would guess it was borrowed. The word's own origin beyond Germanic is uncertain, and it is confused further by parallel forms with p, like Dutch knijp and German kneip. French canif, "penknife," is probably Germanic in the other direction, borrowed from Frankish.
knife (n.) "hand-held cutting instrument consisting of a short blade and handle," late Old English cnif , probably from Old Norse knifr "knife, dirk," from Proto-Germanic *knibaz (source also of Middle Low German knif , Middle Dutch cnijf , German kneif ), a word of uncertain origin. To further confuse the etymology, there also are forms in -p- , such as Dutch knijp , German kneip . French canif "penknife" (mid-15c.) is probably of Germanic origin, perhaps from Frankish. For pronunciation, see kn- . knife (v.) 1865, "stab or kill with a knife," from knife (n.). Intransitive meaning "move as a knife does" is from 1920. Related: Knifed ; knifing . also from 1865
knife came in through Old Norse, and its spelling follows that family's conventions.
What gives it away on the page
| core vocabulary that isn't Old English | Borrowing pronouns is rare and marks deep contact, not fashion.they, them, their, are, take, want, die, knife, window, law |
Placing it without the spelling
| blunt everyday things — and a few words too basic to look borrowed | Old Norsesky, knife, window, law |
Old Norse in the spelling guide · guess another word's family