Why is a jackknife called that did a guy named Jack discover it?

The term is probably derived from "jackleg", meaning "unqualified".

wordorigins.org/index.php/site/jackleg_jackknife no longer exists

I also found a reference that some believe the term may be a nautical reference.

"The origin of the word jackknife comes is unknown but some people have theorized that this type of knife was often carried by sailor(s)(sic) and since "jack" is nautical term for the small flag flown at the "jack staff of a ship" which was also used as a name to call sailors..."

http://references-definitions.blurtit.com/3415109/why-is-a-jack-knife-called-a-jack-knife