"Putting his two cents in"

Let's say, You are in the middle of a heated discussion with, let's say, your teacher. You're just throwing the solid arguments at each other... around the earsthat it bangs and shreds. Just when your teacher seems to be slowly willing to give in, a fellow student absolutely has to get rid of his opinion. And the discussion starts all over again. But he always has to add his two cents! Hm... why actually mustard? Mustard is basically something healthy and natural. But apparently it is mixed up in this statement with something Negative associated.

One might think. The Opposite was, however, the case: mustard was considered in the 17th century - at the time when this idiom took its course - to be an extremely valuable resource. A dish was therefore generally considered to be of particularly high quality and dignity if a blob of mustard could be seen somewhere on the plate and if it could at least be tasted. A few quite shrewd innkeepers converted this unwritten law without further ado to their advantage and quite simply gave every meal, which was ordered in their economy, a touch of the valuable sauce to it. This should give every meal the necessary quality. Stupid only that mustard now by far does not fit to every dish. Just as mustard was served with the meal without being asked, sometimes people express their opinions without anyone wanting to hear them. That is why it is called "putting his two cents in." Kind of makes sense then, doesn't it?