Perhaps, Adler suggests, from a scholion on Aristophanes, Peace 1079, where the poetic adverb touta/kis (sic) occurs in a mock-oracle (web address 1). LSJ s.v. is unhelpful on its sense there; see Platnauer's note.
If it be objected that the present headword is touta/ki, not touta/kis, the source might be Pindar or Callimachus (both of whom use both forms).
[1] cf. (e.g.) epsilon 1904. The present entry's addition of de/ to e)n o(/sw| is presumably nothing more than a reflection of how this phrase is often used in practice.