Sigebert, monk of Gemblours, in 1100, derived this word from Thong-ceaster, the “Castle of the thong,” and says that Hengist and Horsa purchased of the British king as much land as he could encompass with a leather thong. The thong was cut into strips, and encompassed the land occupied by the city of Doncaster.
This is the old tale of Dido and the hide, and so is the Russian Yakutsks. (See Bursa.) Of course it means the “City on the river Don.” (Celtic, Don, that which spreads.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894