Well, 無敵 Muteki means “invincible”, “unrivalled” (lit., “no enemies”). Kenshirō’s 無想転生 Musō Tensei (“Blank Mind Transmigration”, “Rebirth without thoughts”) uses in fact another 無想 musō, that stands for “no thoughts”, “blank mind”.
That’s… complicated, actually. It could be that they wanted to reference Lee’s 表蓮華 Omote Renge (Front Lotus Flower), but the fact is that… 飯綱落とし Izuna Otoshi (Least Weasel Drop¹) is in itself an old move, a staple of almost every fighting game ninja from Claw’s “Izuna Drop” onwards, and of almost every ninja in general after Claw; Claw himself referenced the 1964 manga カムイ伝 Kamui Den, by renowned gekiga artist Sanpei Shirato. By the way, I have read it and it’s nothing short of superb.
The first appearance ever of the Izuna Otoshi happened on the 5th of May 1965, when the monthly manga magazine Garo published the second chapter of Kamui Gaiden, aptly titled 飯綱落とし Izuna Otoshi, whose final fight saw this occur:
The protagonist Kamui mainly used his Izuna Otoshi in aerial fights between high trees where he could catch his opponent in mid-air, but chapter four, むささび Musasabi (16 June 1965), had Kamui fighting in a grass field and therefore demonstrating a variant, still named Izuna Otoshi:
Spoiler
…that’s exactly Guy’s 武神イズナ落とし Bushin Izuna Otoshi.
Kamui Den is the source of many moves and stereotypes in mangas’ and fighting games’ ninjas². The Izuna Otoshi went on to become Claw’s Izuna Drop (often mistransliterated Izna in various SF2 arcade flyers I’ve seen) in 1991, and then many FG ninjas added this move in their arsenal. Then, it appeared as Omote Renge in Naruto, but even the spinning part had already been added by at least 1993, when Hanzō and Galford in the first Samurai Shodown executed the move that way.
¹ There’s a reason for that name. [details=Spoiler]The entire story is narrated in this video, whose translation I’ve summarized.
Later on in the manga, Shirato explained that when Kamui still was a regular Iga ninja and not yet a nukenin, he was studying techniques under his stern master Akame, and became concerned with the fact that his colleague Toera had already learned a powerful move named 山陰 Yamakage (Mountain Shadow), and therefore he had to create a technique that could match that. One day, he observed a runaway horse who was about to kick a child, and sensed there was something strange. Jumping on the horse, he discovered that a small itachi (weasel, also a well known ninja name) hidden in its mane was causing its behaviour. As soon as the weasel had jumped off, the horse calmed down. Amazed that such a small animal could trouble a much larger one, he began to observe weasels, until he saw a prodigious fact. A weasel had jumped on a pheasant’s back, and the pheasant flew away, trying to shake the weasel off its back. But no matter how much it shook, the weasel won’t fall; instead, the weasel managed to wear the pheasant down and make it fall on its head. Thus Kamui began to train with logs until he perfected the Izuna Otoshi. The name “Izuna” comes from the Northern Japan name of the weasel, where they’re named “Izuna” or “Iizuna”. In fact, “Itachi” is the generic term for weasels of all subspecies, and “Izuna” is the more specific Least Weasel. There is also a supernatural being named Izuna as well, but it’s totally unrelated.[/details]
² For instance, his 霞斬り Kasumigiri (Mist Cut) became Kasumi’s 朧 Oboro (Faint) in DOA, and many other techniques in which the attacker seems to “go through” his opponent:
