Source : Webster's Revised Unabridged Dictionary (1913)

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.