If there is a thing that almost all philosophers agree upon, it is the importance of controlling your tongue. Not surprising, really, since philosophers deal mainly in words that they should be very wary of how you speak words and the impact of ill-chosen words.
Tiru is no different. Especially when he says this...
Ondraanun theechchol porutpayan undaayin nandraaga dhaagi vidum - Tirukkural
If but one wrong word causes the smallest harm, all the good wrought by him will turn to evil - Loose Translation
Tiru allows for the fact that a misspoken word may not always be interpreted as causing harm. Thus, if in a negotiation, you use a wrong word, it may not always lead to hurt and its consequences. But, if you DO speak something hurtful AND it causes hurt in the other party, it has the potential to lose you the whole negotiation.
The problem with human interactions is that it almost always seems to exist on the basis of mutual mistrust. Each party strives to trust the other but falls back to distrust more easily than it leans forward into trust. This tendency to mistrust is stronger in some interactions and weaker in others...but, overall, it is easier to cause distrust than it is to gain trust.
AND exactly because distrust is easier than trust, a wrong word spoken at the wrong time causing hurt to the other has the potential to swing the entire mood of the interaction to distrust, thereby destroying the painstakingly built trust of all the work that preceded it.
It is not merely when groups of people are interacting with each other in negotiations. At the individual level, too, you build your friendships by building mutual trust. A person with a loose tongue will find it much more difficult to build and sustain good relationships, precisely because of his tendency to utter hurtful words.
Words are powerful. They need to be used with care.