Written as such, with the double-rho, the headword is attested almost exclusively as the accusative form of skirro/s, a substantive used by Galen (e.g. 11.736.11) and other medical writers for certain types of tumors and indurations (although for skirro/s = 'chalk', see sigma 625, sigma 617, and under epsilon 1263). Yet there is great variation in manuscripts between readings with double-rho and single rho (even here in the Suda, Adler reports, mss GM read skiro/n rather than skirro/n); so it is possible, if not probable, that the lexicographer has in mind a form of ski=ros ('dry land', etc.), skiro/s ('cheese rind', on which see sigma 621), or some related term.
See also sigma 566, sigma 623, sigma 624, sigma 628; Hesychius sigma892, sigma893, sigma962.
Adler cites Lexicon Ambrosianum 546 by way of contrast.