זיך

From Middle High German sich, from Old High German sih, from Proto-Germanic *sik, which see for cognates.

Some German dialects use sich as a general reflexive of the plural. This usage clearly started out from the Middle High German accusative variant unsich (“us, ourselves”), as it is more widespread for the 1st person plural than the 2nd person. However, no dialect seems to allow sich for the 1st and 2nd persons singular. At least this part of the Yiddish development was most likely influenced by descendants of Proto-Slavic *sę, *sebe.

זיך • (zikh)

1. Reflexive pronoun for all persons and numbers: itself, himself, herself, oneself, themselves, myself, ourselves, yourself, yourselves.
2. (Hungarian Yiddish, New York Chassidish) Reflexive pronoun for the third person only: itself, himself, herself, oneself, themselves.