Jack knife or “jackleg knife” is a term often used in the 1700’s or 1800’s to describe a generalized pocket knife, while variants of this term have been used as far back as the 1670’s.
The term “jack” (in early times) was an American southern slang adjective meaning unskilled or indiscriminate. This lent itself to referring to general, unskilled workers as “jack-leg”, from which (probably) the term “jack of all trades” comes from…among other more commonly known uses of “jack”.
Today, “jack knife” is a term which is describes a less-specialized pocket knife. There are many knives that fall into this generally broad category, which can then be split up into multiple more specialized frame types. (For our purposes, we categorize the standard, general 2-blade [or even 1-blade] knives as jack knives, and then we divide those knives up into more descriptive categories.)