Where does 'ignotum' come from and what does it mean?

Ignotum is the latin word for 'unknown'.

It is found in a motto that was often quoted by medieval alchemists as way of describing their approach.

The phrase is:

Ignotum per ignotius, obscurantum per obscurantius.'

The literal translation might be given as: 'the unknown by the more unknown, the obscure by the even more obscure.'

Another way of putting it would be: a situation where the explanation of a thing is more difficult to understand that the thing itself.