"Ubi Donatus. Proprie calamitatem rustici vocant quod comminuat calamum; h.e. culmen et segetem."
The etymology of its synonym, "disaster," is more direct—δυσ αστηρ, a star of evil influence, or, as we say, "born under an ill planet."
Φιλολογοσ.
Forcellini, s.v. Calamitas, says:—
"Proprie significat imminutionem clademque calamorum segetis, quæ grandine vel impetuoso aliquo turbine aut alia quapiam de causa fit."
He then quotes Servius, Ad Georg, i. 151:—
"Robigo genus est vitii, quo culmi pereunt, quod a rusticanis calamitas dicitur."
Then follows the note of Donatus on Ter. Eun. i. 1. 34.
It appears to me, if "calamitas" were derived from calamus, it would mean something very different from what it does.
Another suggestion is, that the first syllable is the same as the root of cad-o, to fall; l and d, everybody knows, are easily interchangeable: as Odysseus, Ulixes: δακρυον, lacrima, tear, &c. &c. If so, calamitas is a corrupted form of cadamitas. Mar. Victorinus, De Orthogr. p. 2456., says:—