One I’d forgotten from Gibbon. OED reminds me what it means:
< French caducité, as if < Latin *cadūcitātem , < cadūcus : see caducous adj. [ultimately from Latin cadere, to fall]
1. Tendency to fall; quality of being perishable or fleeting; transitoriness, frailty.
1793 W. Roberts Looker-on No. 47. 375 One of those evenings of autumn when the chilling damps of the air, and the caducity of nature, deepen the gloom of a melancholy mind.
2. esp. The infirmity of old age, senility.
1769 Ld. Chesterfield Let. 11 Oct. (1932) (modernized text) VI. 2896 This melancholic proof of my caducity.
1776–88 Gibbon Decline & Fall lxi. (R.) Count Henry assumed the regency of the empire, at once in a state of childhood and caducity. [Bingo!]
3. Roman Law. Lapse of a testamentary gift.
1875 E. Poste tr. Gaius Institutionum Iuris Civilis (ed. 2) ii. 264 The leges caducariæ, which fixed the conditions of caducity.
4. Zool. and Bot. Quality of being caducous.
1881 J. S. Gardner in Nature 26 May 75/1 The spores become detached before germination..this caducity always characterises the microspore.
And small wonder that I forgot it. OED estimates its frequency at almost the lowest level.
This word belongs in Frequency Band 2. Band 2 contains words which occur fewer than 0.01 times per million words in typical modern English usage. These are almost exclusively terms which are not part of normal discourse and would be unknown to most people. Many are technical terms from specialized discourses. Examples taken from the most frequently attested part of the band include decanate, ennead and scintillometer (nouns)…
BTW there’s an excellent article in OED on calculating frequency which I found very instructive.
OED gives examples of words in Band 1, the lowest frequency band, so low that no percntage can be given for frequency of occurrence per million words. See how many you know or can even guess at (I scored a big fat zero.)
abaptiston, abaxile, grithbreach, gurhofite, zarnich, zeagonite.