Speak only words that do no harm
"One should speak only that word by which one would not torment oneself nor harm others. That word is indeed well spoken.
"One should speak only pleasant words, words which are acceptable (to others). What one speaks without bringing evils to others is pleasant."
This one of those situations where the combination of the speaker and the listener decides what is harmful.
In Korea, you can call someone an idiot and everyone laughs, but call someone crazy and them's fightin' words! Where I'm from, it's more the other way around...
I still haven't figured out any definitive answer as to why there's such a difference, but the sarcastic side in me wonders sometimes if perhaps it's that the truth hurts???