Poking around the OED today, I came across this entry, which I’ll reproduce here in full:
aˈcrasial, a. rare-1.
[f. acrasy + -al1.]
Ill-regulated, untempered, intemperate.
1851 S. JUDD Margaret II. xi. 321 (1871) ‘Acrasial Philogamy? Brother Edward, what is that?’ ‘That,’ replied Edward, ‘is an incurable malady to which young persons are subject.’
I guess that the OED included acrasial because of the association of acrasy with Spenser, who has ‘irregularity, disorder, intemperence’ personified in a character named Acrasy in The Faerie Queen. The etymology, though it appears transparently to be a+kratos (i.e. un + mixed or tempered), turns out to be somewhat more complicated according to OED:
[ad. med.L. acrasia, which seems to confuse Gr. ἀκρᾱσία ill-temperature, badly-mixed quality (f. ἄκρᾱτος unmixed, untempered, intemperate) applied by Hippocr. to meats, with ἀκρᾰσία impotence, want of self-command (f. ἀκρᾰτής powerless, without authority, without self-command, incontinent).
Well, the etymology of philogamy is just as transparent, if not more so, but OED didn’t see fit to include that as a headword, despite it having exactly as much usage evidence as acrasial.
In my OED2 (1989), there are 42,383 headwords which are included on the basis of only one citation. Such terms are sometimes called hapax legomena (Greek again, for “once said”), or “nonce words”, which is a term made up by OED editor James Murray for the particular purpose of describing words made up for a particular purpose (as OED3 glosses it: “a word apparently used only ‘for the nonce’, i.e. on one specific occasion or in one specific text or writer’s works.”)
Of the 43,383 single-quotation headwords in OED2, 15,357 (a little more than at third) are from the 19th century, like Sylvester Judd’s immortal, unreattempted deployment of acrasial.
Now, this is very rough, but within these 15,357, I count about 2,300 quotations which, like Judd’s, also contain another word which is not an OED headword. Many of these are spelling variations of one headword or another, or foreign words, or proper or scientific names.
(Even some of these can be puzzling: s.v. corundic, we get the following quote: ‘to express the relative hardness of other substances, by the following parallel terms: Cretic, Gypsic, Marmoric, Basaltic, Felsparic, Crystalic, Corundic’ — but gypsic, felsparic, and crysticalic aren’t OED headwords; and marmoric is included on the strength of only one quotation, but it isn’t this one! basaltic gets three quotes.) [*edit – see 7/11/21 comment by kts]
Some are interesting, especially when the words are being used in parallel ways. Below is a table of haphazardly selected OED hapaxes, from single quotations which also include words not in OED, on the model of ‘Acrasial Philogamy’. Make of this what you will.
I can’t say exactly how many of the 2,300 are like these – what I take to be true “ghost hapaxes”. Perhaps a few dozen, or a hundred or more – but the lines of inclusion get a bit blurry around the edges.