Some interpreters think the illustration has to do with trade, and others think the idea is that someone has defrauded someone else, promised them something, and given them short measure. But other older writers take a different view of the illustration.
What you say is very, very important. Because what you say is what you are going to be judged by. It is no good saying, he is good at heart. Maybe he is but if he has not conquered his mouth, he is going to be judged by what he says. In that rebellion, the only way anybody can figure out who is right and who is wrong is by what they say. What we say is enormously important. Sometimes I think we are carried away with the idea that words are not so important. We can be trivial. We can exaggerate at times. We can be almost dishonest with the people among whom we move at work, in the family, and so on. We can speak out of turn, make unguarded statements, we can say flippant things and foolish things and somehow we think that the Christian heart is going to shine through all this. No, says the parable, you are going to be hanged by what you say. You will wish you had controlled yourself, when you lost your temper, even with somebody close to you, and you were cruel or cutting or unkind. That lives on, that is you. You cannot say, oh, but I am not really like that. Words reveal - or so other people will think - our true character. Words disclose what we really think. If a Christian person rampages about with all sorts of loose and careless speech and worldly speech and covetous speech, think of the influence upon other people and think of the destruction of one's testimony.