Ponzu is a traditional Japanese table condiment or dipping sauce for seafood, steaks, tofu, etc. This citrus sauce blended with soy sauce is frequently used with Shabu Shabu, for the citrus flavor softens the smell of meats and seafood.
The word "ponzu" is made up of two components: "pon" and "su". "Pon" comes from the Dutch word "pons", meaning "citrus juice", and "su" means "vinegar". "Vinegar" here should be interpreted not as merely "vinegar" but "sanbai-su", the equal mixture of vinegar, soy sauce and "mirin". (Mirin is a rice vinegar, or non-alcoholic sake for cooking.) "sanbai" means "three cups". What I mean here is the creator of this condiment chose to blend citrus juice, which is sour, instead of just vinegar.