Quintilian, and J.S. Watson. Institutes of Oratory. Edited by Lee Honeycutt, 2010. Kindle Electronic Edition. Book 8 Chapter 2.
Quintilian urges the proper use of words. Propriety requires not only correct usage, but avoiding being “obscene, or offensive, or mean” (Quintilian VIII.2.1). By “mean” Quintilian intends usage “beneath the dignity of a subject” (Quintilian VIII.2.2.). This does not require avoiding common terms for ideas, but using appropriate dignity in speech (Quintilian VIII.2.2(. Being adequately specific is also important (Quintilian VIII.2.7). Quintilian advocates use of characteristic words and word groups which fit together well, as in an epithet (Quintilian VIII.2.10).
Obscure language use can come from use of vague or obsolete words, as well as regional dialect (Quintilian VIII.2.12-13). Excessive use of compounds creates obscurity (Quintilian VIII.2.14), as do digressive comments (Quintilian VIII.2.15). Ambiguous wording is also a serious problem in writing and speaking (Quintilian VIII.2.16). Some authors seek brevity in such a way as to create obscurity (Quintilian VIII.219). This is a difficulty, again, because the speech is not clear. Quintilian says the worst obscurity uses plain words with another hidden meaning (Quintilian VIII.2.20).
In sum, Quintilian values clarity in language and expression. It is only through clear speech that an audience will understand and accept the orator’s arguments (Quintilian VIII.2.22).