Sampi or Disigma is a mysterious Greek symbol. Its meaning is still debated today. Some believe it to be a replacement of 'san'. Before that, it was a letter that was used in the Greek alphabet in a few Greek cities in the 5th century BC. At that time, it looked like 
Sampi
Source
The origin of sampi is unclear. It could have originated either directly or as a further development of the archaic Greek letter San (Ϻ) from the Phoenician Sade ( 
). The sade in Phoenician denoted the emphatic s sound. The San was derived from the Sade and was used in some variants of the Greek alphabet instead of the Sigma for the /s/ sound.
However, the Sampi is not placed in the alphabetical order at the position of the Sade or San between Pi and Qoppa, but is at the very end after Omega. This would rather indicate that the Sampi was a later neologism.
The name Sampi is also not sufficiently clarified. It could be related to the name San. Another possibility is that it goes back to ὡσὰν πῖ hōsàn pî "like Pi" and refers to the external similarity between Sampi and Pi.
Use as letter
In some alphabets of Ionia (Teos, Ephesus, Kyzikos and Halikarnassos) and Pamphylia (Perge, Sillyon) a special letter with the form
resp. 
occurs. This is probably the forerunner of the Sampi. This character is found in the 6th to 5th century B.C. in Ionic and in the 4th to 2nd century B.C. in Pamphylian inscriptions and is found in places where most dialects have σσ (ss) and classical Attic has ττ (tt), e.g. ΘΑΛΑ Α instead of ΘΑΛΑΣΣΑ (thálassa) or ΘΑΛΑΤΤΑ (thálatta). How this combination of letters was pronounced in antiquity is disputed. However, since it goes back linguistically historically to *kj, the pronunciation variant /ts/ seems possible. The fact that the sampi was later replaced by σσ could then indicate that the pronunciation had changed from /ts/ to /ss/.