PIE base pronoun of the third person and reflexive (referring back to the subject of the sentence), and as s(w)e- further appearing in various forms referring to the social group as an entity, ‘(we our-)selves’.
Derivatives include sever and secret ‘known to oneself’, from Latin secretus ‘separate, set apart’, from secernere ‘to separate, part’, from se-, word-forming element ‘without, apart, aside’, probably originally ‘by one’s self, on one’s own’, and cernere ‘sift, distinguish, separate’.
[Pokorny *se- ‘self, one’s own’, pp. 882–884.]