The headword appears to be an uncontracted aorist subjunctive of i(/sthmi, third person plural, glossed with the corresponding contracted form. This uncontracted form is unattested outside lexica. (In a fragment of Antimachus quoted by Athenaeus, Deipnosophists 11.468A [11.33 Kaibel], the dative plural perfect participle e(sthw=si occurs.)