It really depends on what you mean by "originally". We can trace the word back at least as far as a Proto Indo-European form "skei" meaning 'cut', 'split' (i.e. about 5,000-6,000 years ago). This developed into various Latin words, including "scindere" (from which we get the English word "rescind", and which has become "scinder" in French still meaning something like 'to divide'), and also 'scire' ("to know" < "to split/distinguish truth from flasehood"). In Latin, 'scire' could be combined with the perfix 'ne-' ("not"), to give 'nescire' ("to not know about", "be ignorant of"), leading to the adjective 'nescius' ("ignorant"), which was the word eventually borrowed into English and which then gradually changed its meaning.
So if your notion of "original" is "origionally appeared as a word that was ostensibly 'English'", then the meaning was probably close to 'ignorant', 'foolish'. But the origin of the word can be traced back much further than that. But then, there's nothing hugely magical linguistically speaking about Proto Indo-European (or Latin, for that matter)-- they're just arbitrary points (or rather, vaguely-bounded time periods) in history when we're able to posit something about the form of language that existed. Presumably the Indo-European form "skei" that we believe existed around 6,0000 years ago itself developed from something, but we can't say exactly what. So in a strict sense, we can't say that this or anything else was the "original" form/meaning.
This is why arguments of the "well, this word really means X because that was the original meaning" type are generally spurious.
As to why it changed its meaning: well, in a sense, this word hasn't done anything remarkable. It's the normal state of affairs for words to change their meanings over time, and largely the more remarkable cases are words that have (e.g. because they're highly technical words usually consulted alongside their original, arbitrary intended interpretation whenever they are used) retained their 'original' interpretation.