Daikon oroshi might be described as the Japanese answer to parsley. It is a popular garnish served in Japanese restaurants. It consists of grated bits of the daikon radish. Daikon oroshi is often mixed with sauces. The device used to grate the radish is called an oroshigane (おろし金).
Closely related to daikon oroshi is the word tsuma, which is a more general term for garnish. This word is the source of a common Japanese metaphor: sashimi no tsuma (garnish for sashimi) Sashimi (thinly cut strips of raw fish) is often served with a garnish such as daikon oroshi. However, the garnish itself is of little consequence.
Therefore, if someone says that his presence at a meeting was “garnish for sashimi” he means that his presence was of little meaning for the meeting. This metaphor is often used as a tongue-in-cheek method of self-deprecation.