Being the nickname for the judoka Ryoko Tani (谷亮子 Tani Ryouko). According to Wikipedia (where else?), it comes from the name of a popular series of manga about judo.
The next question then is, why is she soooooooooooooo popular? That includes the ultimate compliment for any celebrity, starring more than once in the manga Urayasu Tekkin Kazoku (“English” title Super Radical Gag Family)