An Etymological Dictionary of the German Language/sein
sein, poss. pron., ‘his, its,’ from MidHG. and OHG. (also OSax.) sîn; comp. Goth. seins, ‘his’; allied to Goth. si-k, ‘himself,’ formed with the poss. suffix -îna- like mein and dein. Comp. sich; the further discussion of the word belongs to grammar.
sein, anomal. vb.; its tenses are formed from various stems. The Teut. prim. stems are es, -s, with the same meaning (OHG., MidHG., and ModHG. ist, OHG. and MidHG. sint, ModHG. sind; subj. mood, OHG. and MidHG. sî, ModHG. sei; inf. MidHG. sîn, ModHG. sein; comp. Goth. 3rd pers. sing. ist, plu. sind; optat. sijau; AS. and E. 3rd pers. sing. is, 3rd pers. plur. AS. sind); corresponding to the Aryan root es in Lat. es-t, Gr. ἐστί, Sans. ás-ti, Lat. sunt, sîm, Sans. sánti, &c. The second stem begins with b, ModHG., MidHG., and OHG. bin, OSax. bium, AS. beó, ‘I am’ (AS. also ‘I shall’), connected with the stem of Lat. fio, Gr. φύω, Sans. bhû, ‘to become.’ For the third stem (of gewesen and war) see under Wesen. Further details belong to grammar.