Prelapsarian is an adjective. Strictly speaking it means of or relating to the period before the fall of Adam and Eve; characteristic of the time before the Fall of Man. It is, however, also used in a broader context to mean innocent and unspoiled; or simply ancient.
In a Ruth Rendell novel, for example, a young girl who drives an automatic car describes her mother's car with gears as prelapsarian.
It appears to be pronounced prlps-ARE-ian but don't quote me on that - I'm no good at working with the phoentic alphabet.