The headword is a two-ending adjective in the neuter nominative/vocative/accusative (and masculine/feminine vocative) singular; see generally LSJ s.v. thmelh/s.
The earliest attestation of the headword form is at Phrynichus, Praeparatio sophistica fr. 356.1, the immediate source of this and other lexica: see n. 1 below. (There is but one instance outside lexicography: Nicetas Choniates (C12-13 CE), History Alex. II, 254.11.)
cf. generally tau 504, tau 505, tau 506.
[1] The gloss is the same form as the headword. The headword is identically glossed by Phrynichus loc. cit., Photius' Lexicon, the Synagoge, ps.-Zonaras 1728.22, and Lexica Segueriana 387.1. The lemma is similarly glossed by Hesychius.
[2] The same supplementary lemma and glosses are given by Lexica Segueriana 387.2.