This adjective is one of four — the others being erstwhile, quondam and umquhile — with the same meaning: formerly, in the past. All are unfamiliar and un-English in appearance, though whilom is not the least odd. None is common but whilom is rare enough today to the extent that current dictionaries mark it as archaic. This may not be a surprise to the writer of this letter, an expatriate professor of English and Irish Literature in a German university:
As a whilom Irishman since 1961, I follow the news rather seriously.
The Irish Times, 18 May 2010.
It was once part of the vocabulary of literate English speakers, such as J M Barrie, who wrote in The Little White Bird of 1903: “Whom did I see but the whilom nursery governess sitting on a chair in one of these gardens”. He meant that that the lady had once been a governess, but was one no longer. Barrie wrote that at what proved to be the peak of the word’s popularity. By the time of this next appearance, it had been rapidly falling away from favour for a couple of decades:
His might have been, as his whilom playmate, the Hon. Galahad Threepwood, had made so abundantly clear in Chapters Four, Seven, Eleven, Eighteen, and Twenty-Four of his immortal work, a frivolous youth, but in his late fifties he was taking life extremely seriously.
Heavy Weather, by P G Wodehouse, 1933.
The word dates to Old English, at a time when the language was heavily inflected — adjectives, nouns, and verbs taking different endings depending on the job they were doing. Whilom — then spelt hwilom — was the dative plural of hwil, the same word as our modern while. As English progressively lost its inflections, the word became a fossil, with its ending stuck to it permanently; at the same time the meaning shifted to mean something of a former time, a change that was complete by the fifteenth century.