Jef wrote:

I had to lol at the quote.

"To swear, except when necessary, is becoming to an honorable man." —Quintilian

Given the time frame in which he lived and the obvious context, I have little doubt that by "swear," Quintillian meant "swearing an oath." I'd be willing to bet lots of money that he in no way intended it to be a comment on the use of profane language.

"the obvious context" here includes the phrase "except when necessary." how do you explain this portion of the context as it relates to your own suggestion? what would this part of the context have to do with "swearing an oath?" that one should swear an oath, except when necessary? not much clarity in that suggestion. surely, a noted orator such as he would have just begun "To swear an oath..." to provide little doubt of his meaning...

to be fair, the exception also isn't exactly clear as it relates to profanity. after all, why would someone curse at all other times "except when necessary?" perhaps this is early vs. modern linguistics.

what we do know is that Quintilian was a rhetorician -- he expressly dealt in oratory and the use of words. he wanted to influence change to the venacular of the day which was held prominent by the likes of Seneca. he was almost certainly discussing profanity here.

looking at other quotes by him also show his concerns of vices and virtues related to oratory and use of words:

http://www.brainyquote.com/quotes/authors/q/quintilian.html

"It is the nurse that the child first hears, and her words that he will first attempt to imitate."

"God...has impressed man with no character so proper to distinguish him from other animals, as by the faculty of speech."

but your suggestion isn't all lost: according to some researchers, turns out profanity comes in two main flavors -- oath and obscene.

http://newsfeed.time.com/2013/04/10/nine-things-you-probably-didnt-know-about-swear-words/

good wiki on Quintilian too...