It did make a /kn/ sound once. Most words spelt kn- came into English from Norman French; "knife" - once spelt "knyf"- is related to the modern French canif.
Norman language - Wikipedia, the free encyclopedia
English was formed from so many different languages, and has borrowed from so many others, that the spelling can seem confusing. But "kn-" words are quite consistent; the "k" is just a written fossil; the sound is /n/ .