The headword is a two-ending adjective in the masculine/feminine nominative singular; see generally LSJ s.v.
The entry is perhaps generated by one of the many instances of the neuter accusative singular pro/wron, first attested (C1 BCE - C1 CE) in Philo Judaeus, De specialibus legibus 3.91.1: e)/ti kai\ pe/nqos pro/wron a)nede/canto, yet they too experienced untimely grief. In lexicography note also Hesychius pi4109, s.v. pro/wron with gloss pro\ kairou=: prematurely. Epigrams are another possible source: see e.g. Greek Anthology 7.643.3-4 (web address 1), an epitaph for a slave girl attributed to Crinagoras; ti/ pro/wron e)fi/eis moi=ran th=| pa/ntws sei=o/ e)ssome/nh|;, 'why do you thrust an untimely fate at her who in any case will at some time be yours?'; Gow and Page, pp. 208-9.
The first instance of the exact form (masculine/feminine nominative singular) of the headword is considerably later (C4 CE), in Oribasius, Collectiones medicae 39.9.1: ou)de\n ga\r ou(/tws e)gkoptiko\n ei)s e)pi/dosin yuxh=s kai\ sw/matos w(s h( pro/wros kai\ dayilh\s xrh=sis tw=n a)frodisi/wn, 'for nothing is so detrimental to the nurture of soul and body as the untimely and excessive enjoyment of sexual pleasures'.
[1] Coincidentally, this glossing phrase occurs in Xenophon, Oeconomicus 20.16 (web address 2): kai\ a)/llos ge a)nh\r diafe/rei tw=| pro\ th=s w(/ras a)pie/nai, 'and another man does make a difference in disengaging before the appropriate time'.