(n.) nineteenth-century-like style or characteristics
And that, according to the Oxford English Dictionary, makes a decemnovenarian a nineteenth-century gentleman (or a man who adopts nineteenth-century characteristics), while to decemnovenarianize is to behave like a decemnovenarian.
As some of you clever, clever people pointed out, all of those words are based straightforwardly enough on decemnovem, a Latin word for the number 19; it in turn is formed from the Latin for “ten”, decem, plus the Latin for “nine”, novem. And yes, those are the same roots as those of November and December, which were once the ninth and tenth months of the year.
Appropriately enough, the word decemnovenarian was coined in the nineteenth century by an English mathematician and logician named Augustus De Morgan. As well as formulating and giving his name to De Morgan’s Laws, a pair of fundamental rules in the study of logic, De Morgan was a spiritualist investigator who alongside his wife, Sophia, dedicated many of his final years to the study of the supernatural.
In 1863, Sophia published a groundbreaking account of their decade-long investigations entitled From Matter To Spirit, for which De Morgan—writing under the pseudonym “A. B.”—wrote the preface:
We, respectable decemnovenarians as we are, have been so nourished on theories, hypotheses, and other things to be desired to make us wise, that most of us cannot live with an unexplained fact in our heads. If we knew that omniscience would reveal the secret in a quarter of an hour, we should in one minute have contrived something on which to last through the other fourteen. The commonest of all questions is, “How do you account for...?” and woe to him who, not having an answer of his own, shall refuse to accept that of the querist.
From there, the word decemnovenarianism appeared in print for the first time the following year in an 1864 article by the women’s suffrage campaigner Frances Power Cobbe, and the verb decemnovenarianize followed that in 1890.
And ironically—or perhaps fittingly—all three have long fallen out of use since the turn of the twentieth century.