Re: etymology, I was once told or read, perhaps in law school, that this was a corruption of the Latin phrase for mental defect or unsound mind, "non compos mentis."
but see
Quinion and here is OED online:
Etymology: Origin uncertain; the second element may perhaps be poop v.2 (compare earlier noddypoop n.). The first element in α forms may show a form derived from a name; compare French nicodème simple or naive person (see Nicodemite n.). With the first element in β forms compare ninny n.1
Various other etymologies have been suggested. Johnson's suggestion of
non compos n. does not agree with the earliest forms.