Laudator Temporis Acti

I just encountered theta and zeta as nouns in Walter of Wimborne's Ave Virgo Mater Christi, in his Poems, ed. A.G. Rigg (Toronto: Pontifical Institute of Mediaeval Studies, 1978), pp. 146-147.

Theta isn't in Alexander Souter, A Glossary of Later Latin to 600 A.D. (Oxford: Oxford University Press, 1949), Albert Blaise, Lexicon Latinitatis Medii Aevi (Turnhout: Brepols, 1975), or J.F. Niermeyer, Mediae Latinitatis Lexicon Minus (Leiden: E.J. Brill, 1976). Rigg ad loc. refers to Isidore of Seville (560-636), Etymologies 1.3.8 (tr. Stephen A. Barney et al.):

There are also five mystical letters among the Greeks. The first is ϒ, which signifies human life, concerning which we have just spoken. The second is Θ, which [signifies] death, for the judges used to put this same letter down against the names of those whom they were sentencing to execution. And it is named 'theta' after the term θάνατος, that is, 'death.' Whence also it has a spear through the middle, that is, a sign of death. Concerning this a certain verse says:

How very unlucky before all others, the letter theta.

Quinque autem esse apud Graecos mysticas litteras. Prima ϒ, quae humanam vitam significat, de qua nunc diximus. Secunda Θ, quae mortem [significat]. Nam iudices eandem litteram adponebant ad eorum nomina, quos supplicio afficiebant. Et dicitur Theta ἀπὸ τοῦ θανάτου, id est a morte. Vnde et habet per medium telum, id est mortis signum. De qua quidam:

O multum ante alias infelix littera theta.

Walter of Wimborne used theta in the phrase theta tetre mortis, i.e. the death of death.

Zeta, on the other hand, is in all the dictionaries just cited, with the meaning diaeta = house.