LSJ s.v. notes this headword, a)ciepai/netos, as a variant reading for a)cie/painos in Xenophon Hellenica 4.4.6 -- which in fact has the feminine genitive singular, i.e. a)ciepainota/ths or a)ciepainetwta/ths (referring, either way, to death). That that passage does lie behind the present entry is corroborated by Hesychius s.v. a)ciepaino/tatos, glossed a)/cios e)pai/nou; see Latte ad loc.