The headword is nominative plural of the feminine noun sh=ragc (sigma 335 and see generally LSJ s.v.), first attested in Sophocles and Plato. This nominative plural, indeed, might be extracted from Plato, Phaedo 110A, a passage quoted in Timaeus' Platonic Lexicon and in Stobaeus' Florilegium. Even if so, however, the glossing material -- paralleled in other lexica: see next note -- seems to have been generated by a much later passage, with accusative plural, in Gregory of Nazianzus, Orations 4.30 (PG 36.1213c); see the scholia there.
[1] Same or very similar glossing -- the opening adjective is sometimes e)pimh/keis (as here), sometimes u(pomh/keis -- in the Synagoge (sigma48), the Lexicon haimodein (sigma 24), and Photius' Lexicon (sigma182 Theodoridis); cf. also e.g. Etymologicum Gudianum 499.44-47, and Etymologicum Magnum 711.34-38 (Kallierges).
[In her critical apparatus Adler reports that ms A reads oi)=on: some alone being; also that ms G transmits the masculine present active participle u(potre/xwn, running within, whereas the neuter is required, since the governing substantive is to\ u(/dwr, the water; and that ms F omits the definite article to\ before u(/dwr.]