In English, most nouns of three or more syllables are proparoxytones, except in words ending in –tion or –sion, which tend to be paroxytones (operation, equivocation).
In medieval Latin lyric poetry, a proparoxytonic line or half-line is one where the antepenultimate syllable is stressed, as in the first half of the verse "Estuans intrinsecus || ira vehementi."
He is commenting on this passage from Smaragdus of Saint-Mihiel's didactic poem on grammar: Here is Curtius' note: Sad is the lot of the interjection, for of all the parts of speech it has the lowest place.
Mallarmé was so touched by this that he wrote a prose poem on the "Death of the Penultimate" (Le Démon de l'analogie in Divagations).
It ends: Je m'enfuis, bizarre, personne condamnée à porter probablement le deuil de l'explicable Penultième.