The headword is nominative singular. This particular form is attested only in lexicography; cf. Pollux 4.105.
[1] Future active infinitive of the verb κυβιστάω ('to tumble': kappa 2598), from which the headword is a nominal derivative. This particular form may be adduced here only because it has the same beginining as the headword (κυβιστησ- ). It is attested at Plutarch, Moralia 1047F (= Chrysippos fr.688).