A slipjoint is a "pocketknife", with 1 or more blades. The blades don't lock because there is no locking mechanism. They stay open or closed by the pressure exerted on the blade by a backspring.
Slipjoints are the kind of knives your dad or grandad carried.
They're still made today by companies like Case, Queen, Schatt & Morgan, Canal Street Cutlery, and a few others.