The name is a jargon of sushi restaurant where they call soy sauce "murasaki," vinegared rice "shari," or green tea "agari," thus it is not originally a word to be used by customers; however, it has become popular as a common noun.
いずれも醤油をムラサキ、酢飯をシャリ、茶をアガリと呼ぶのと同じ寿司屋の符牒であるため、本来は客が使うべきではない言葉であるが、普通名詞として一般化している。