[1] The headword is aorist active of prosterni/zw, first person singular, augmented as if the prefix is pro-. LSJ refer instances of prosterni/zomai to prossterni/zomai, meaning that the prefix is (as the sense requires) pros- (and the verb is cited only in the middle voice). The corresponding entries in the Synagoge and the Lexica Segueriana spell with double sigma prosste/rnisa (and have the same gloss). If this is the form transmitted by the lexicographical tradition, the Suda compiler may have attempted to emend it by changing the first sigma to an epsilon as the augment. But in this case the transmitted form remains unexplained. (At Photius, Lexicon pi1363, Theodoridis prints the headword as proste/rnhsa, obelized, and follows Alberti and Cobet in regarding it as corrupted from prosesternisa/mhn.)
[2] Aorist middle participle in the masculine nominative singular, attested with single or double sigma in Josephus and later writers. See also e)nsternisa/menos at epsilon 1443.