Allan Metcalf has a Lingua Franca piece on the word equinox which is mildly amusing but which I wouldn’t bother posting here except for this bit: “So getting back to equinox: The first observation is that there are two exquinoxes (yes, that’s the plural) every year…” My first reaction was “No, that isn’t the plural, what the hell are you talking about?” But of course I’ve learned not to trust first reactions, so I looked it up and confirmed my sense of things: not only is there no mention of such a plural in my various dictionaries (though there is an alternative plural equinoctes), but an Advanced Search of the OED produces the unequivocal “No results found for ‘exquinoxes’.” In other words, this silly form does not occur anywhere in the text, citations, or etymologies of the most comprehensive dictionary of English. So I was about to write an indignant letter to Allan Metcalf saying “What the hell are you talking about?” … but I decided to make one last try and search Google Books, and to my amazement there were pages of results like “The Ouse, or Isis, as Sir Henry Spelman says is its proper name, ‘is remarkable for its extraordinary over-flowing’ at the two Exquinoxes” (1781), “Great atmospherical commotions also excite and exasperate them, and hence they are more obstreperous, and require more care at the period of the exquinoxes than at any other time” (1839), “The port of Tripoli is anything but a safe one, the rottenness of its bottom rendering the anchorage very unstable, especially during the tremendous gales that blow there during the exquinoxes” (1878), “that anyone should have mixed up three arbitrary methods of determining the exquinoxes of a planet” (1902), etc. They’re virtually all from the nineteenth or very early twentieth centuries, but there are recent outliers, e.g. “FE and HE are the group means for vernal and autumnal exquinoxes, respectively” (1973) and “The exact dates of the Vernal and Autumnal Exquinoxes may vary by a day or two” (2001). Does anybody have any idea what might be going on here? How was this misbegotten form invented, and how did people get the idea it was a good thing to keep using?
Update. After all that, it tunrs out that “exquinoxes” was in fact a typo as some commenters suggested, and Prof. Metcalf has asked his editors to correct it. Ah well, we all had fun guessing!