Autokey cipher
A polyalphabetic cipher whose key is extended with the plaintext itself, so it never repeats.
The repeating keyword was the Vigenère cipher's weak spot: the Kasiski examination finds its length, and frequency analysis does the rest. Autokey fixes that by using a short primer and then the message itself as key. Ironically, this is what Blaise de Vigenère actually described in 1586 — the cipher that carries his name today is a simpler scheme he did not invent.
The flaw is that the key is the message. An attacker who guesses a likely word anywhere in the text can decrypt fragments and extend them in both directions. The idea lives on in modern stream ciphers, where ciphertext feedback drives the keystream.