The headword (already in Hesychius lambda35 but with a different gloss: to\ mh\ nasto/n, "what is not firm") is the neuter nominative/accusative singular of this adjective, possibly though not necessarily extracted from the quotations given from the Greek Anthology. The elusive sense of this adjective resists any firm representation by an English equivalent.
cf. sigma 1733.
[1] Quotation (from late historiography) unidentifiable.
[2] From an erotic epigram by Paul the Silentiary (originally a sort of head usher in the imperial palace, who enforced quiet, but by the sixth century CE an official of senatorial rank): Greek Anthology 5.264.6 (not Adler's '263'). For Paul, to whom about eighty epigrams are ascribed, see generally OCD(4) s.v. Paulus (p.1096). (The Anthology is abundantly represented in the Suda: see Cameron 278-282).
[3] Philip of Thessalonica was an epigrammatist of the first century CE and himself an editor of an anthology, lost to us but embodied in the Anthology, where some eighty of his epigrams appear. Consult OCD(4) s.v. Philippus(2) (p.1163); Gow 15; Cameron 15. The verse in question, 6.231.3 in the Anthology's dedicatory epigrams, consecrates a sacrificial cake to Isis; but it accords ill with the sense the Suda ascribes to the adjective. W.R. Paton, the Loeb translator, renders it 'crumbling', but 'flat' seems more likely from comparison with the application of the adjective to a tortoise's shell in Philostratus Maior, Imagines 1.10.2. See further extracts from Greek Anthology 6.231 at alpha 4223, epsilon 1499, kappa 1222, kappa 1310, mu 447, and psi 31.