~ugly~, which signifies the reverse of beautiful or want of comeliness (actual or figurative) is colloquially extended in the United States to uncomeliness of character or personal demeanor; as an _ugly_ fellow; an _ugly_ beast; anger makes him _ugly_. In polite speech this usage is not sanctioned. Say “irritable,” “vicious,” “quarrelsome,” as the disposition inclines or indicates.
~un-~: For the sake of lucidity the use of a negative prefix with a negative antecedent should be discouraged. Avoid such expressions as “He spoke in _no un_mistakable terms” which means, of course, “mistakable terms” the direct opposite of the speaker’s intention. “_Not_ an _un_kempt one among them” means that all were well kempt.
~unbeknown~: A vulgar provincialism used chiefly in the form _unbeknownst_.
~uncommon~: Used for _uncommonly_: a vulgarism meaning “to an unusual degree or extremely.” Do not say “Her eyes are _uncommon_ beautiful”; say, rather, “... _uncommonly_ beautiful.”
~unconscionable~: When used for _unconscionably_ is a bad provincialism. Used also by the illiterate instead of _uncommonly_; as, “She is an _unconscionable_ handsome girl”--this is bad English.
~under~: Much philological nonsense has been written in disapproval of the expression “_under_ his signature,” for which “over his signature”--that “preposterous conceit,” as Gould aptly terms it--is suggested as a substitute. But it is clear that the expression is elliptical, and means “under sanction or authority of his signature.” “_Under_ oath” is good enough to impress upon an unwilling and prevaricating witness the distinction between perjury and a lie, and that although he does not physically lie _under_ the oath.
~understand~ should not be used as an expletive with interrogatory inflection, as a contraction of “_Do you understand?_” There is no excuse for this nor for its objectionable iteration. Avoid such absurdities as: “Grammar, _understand_, is the science that treats of the principles, _understand_, that govern the correct use of language,” etc. _See_ is also misused in the same manner.
~unique~: As this word implies “being the only one of its kind” it should never be preceded by “very” which implies degree. On this subject the STANDARD DICTIONARY says: “We may say _quite unique_ if we mean absolutely singular or without parallel but we can not properly say _very unique_.”