An inseparable prefix, or particle, signifying not; in-;
   non-. In- is prefixed mostly to words of Latin origin, or else to words
   formed by Latin suffixes; un- is of much wider application, and is
   attached at will to almost any adjective, or participle used
   adjectively, or adverb, from which it may be desired to form a
   corresponding negative adjective or adverb, and is also, but less
   freely, prefixed to nouns. Un- sometimes has merely an intensive force;
   as in unmerciless, unremorseless.