Tomoe-nage is often translated as "stomach throw" or "circle throw", but these are not direct translations. What does tomoe mean, and why is the throw called this?
Tomoe (巴) refers to a circular anti-symmetric symbol commonly found in Japanese heraldry. The usage in the judo throw specifically refers to the two-tailed version, futatsu-domoe:
This is very similar to the Chinese taijitu ("yin-yang" symbol) ☯, or Korean taegeuk (famously appearing on South Korea's flag) 🇰🇷.
Tomoe-nage is named as such because Kano thought the positioning/movement of tori and uke's bodies resembled this symbol during the throw:
The origin of tomoe-nage
Randori techniques resembling tomoe-nage are found in the jujutsu textbook Shikatsu Jizai, Sekkotsu Ryoho, Jujutsu Seirisho as sutemi-tori and tachi-sutemi-tori.
Master Kano bestowed this technique with the name tomoe-nage. In Judokai chabanashi (from Judo, published by Judo Kai Honbu, 1916), it says:
"Master Kano was an expert at tomoe-nage. This was originally a Tenjin Shinyo-ryu technique named Ando-gaeshi, after an certain Ando who was skilled in the technique.
It was at that time probably named along with uki-waza, or called Kano-nage or Kano-gaeshi.
Master Kano once said that he named this technique after its resemblance to two objects moving in a circular form"