Old English, of Germanic origin; from an Indo-European root shared by Latin in- and Greek a-.
The prefixes un- and non- both mean ‘not’, but there is often a distinction in terms of emphasis. un- tends to be stronger and less neutral than non-: consider the differences between unacademic and non-academic, for example ( his language was refreshingly unacademic; a non-academic life suits him).
Definition of un- in:
- The US English dictionary