The headword is the genitive singular form of ski/pwn, and has a handful of literary attestations, e.g. Greek Anthology 7.457.1 (Ariston). Similar glossing for the nominative form at Hesychius sigma1022. The Suda's sigma 578 is an entry for the same noun (though nominative there) with the alternative spelling skh/pwn, which would have been homophonous in late Greek. Adler reports that this alternative spelling is introduced in this entry by a corrector to ms A, but that a marginal note by the learned owner of ms M insists that it be written with an iota (rather than eta). Adler also adduces Lexicon Ambrosianum 471 for the lemma.
[1] This alternative is presumably quoted from somewhere but not attested outside lexicography.