Those which have acquired an opposed or contrary, instead of a merely negative, meaning; ; or else an intensive sense more than a prefixed not would express.
Those which have the value of independent words, inasmuch as the simple words are either not used at all, or are rarely, or at least much less frequently, used; as, unavoidable, unconscionable, undeniable, unspeakable, unprecedented, unruly, and the like; or inasmuch as they are used in a different sense from the usual meaning of the primitive, or especially in one of the significations of the latter; as, unaccountable, unalloyed, unbelieving, unpretending, unreserved, and the like; or inasmuch as they are so frequently and familiarly used that they are hardly felt to be of negative origin; as, uncertain, uneven, and the like.
Those which are anomalous, provincial, or, for some other reason, not desirable to be used, and are so indicated.
Un- is prefixed to nouns to express the absence of, or the contrary of, that which the noun signifies; as, unbelief, unfaith, unhealth, unrest, untruth, and the like.