On the one hand, it sounds like you're looking for etymology - how our current words have evolved over time from other languages and roots. Here's the entry for "knife" on the Online Etymology Dictionary. http://www.etymonline.com/index.php?term=knife&allowed_in_frame=0
The Story of English is a great book/video series about how English has changed over time, thanks to Roman, Saxon, Viking, and French invasions, as well as the continual borrowing from other languages since then. It's particularly helpful to see why English often has two or more words for the same thing (often because Saxons used a Germanic word for it and the French conquerors used a French word for it, and both have become common in English). But ultimately you're going to get to the end of the etymology chain and it's going to say "from the Proto-Germanic knibaz of uncertain origin" (that's knife) or "from the Greek whatever" and there's no info before that. If the question is more existential than "where did this English word come from" I'm not sure how to answer that. Sometime back before recorded history someone started calling a thing some name and it stuck and everyone else started using it, and over time it became the word we know today - and of course, in other parts of the world, things have different names because either their ancestors came up with different names initially, or because their languages have evolved differently over time from a common source.