"(of) his;" "(of) hers;" "(of) theirs;" "(of) yours"
Old Spanish súo. From Latin suus 'id.'
From Proto-Italic *sowós. From Proto-Indo-European *seu̯ós 'id.,' from *su̯e "self" (whence se).
"This word *swe has given rise to an adjective indicating “personal belonging”: Skt. sva-, Lat. suus, Gr. *swós (*σwός). We must note that *swos is not in Indo-European the pronoun of the third person singular, as might be supposed by the relation of Lat. suus to meus and tuus. We instinctively make suus the third term of the series. Just as we put I, you, he in the verbal inflexion, it seems normal to us to have the pronominal series my, your, his. The relationship of these forms was quite different in Indo-European: *swos is the reflexive and possessive pronoun equally applicable to all persons." ~ E. Benveniste, Indo-European Language and Society (1973)