The Collaborative International Dictionary of English v.0.48:

Tautoousian \Tau`to*ou"si*an\, Tautoousious \Tau`to*ou"si*ous\,
   a. [Gr. ?; ?, for ? ? the same + ? being, essence.]
   Having the same essence; being identically of the same
   nature. [R.] --Cudworth.
   [1913 Webster]