the mind > attention and judgement > testing > attestation, witness, evidence > [adverb] > as testimony (3)
in witness: as a testimony or piece of evidence. Now rare or Obsolete except as in sense 3.
The action of bearing witness or giving testimony. †in witnessing of, as a witness to; †to bear witnessing, to bear witness.
The Latin word teste in ablative absolute constr. with a pronoun (e.g. meipso myself) or name of a person, as used in the authenticating clause…