Word Choice – Why and for Whom Does ‘Unbeknown’ Become ‘Unbeknownst’?

Always taking a bit of a chance using the Internet to answer an Internet question, but Daily Writing Tips says that not only are while and whilst interchangeable, but that in fact while is the original version. The very authoritative Michael Quinion backs this up on World Wide Words (and that is a site well worth visiting for anyone interested in the English language).

So I think your question proceeds from a false premise: they haven't come to be used to mean the same thing, they do mean the same thing, and there is nothing wrong with using while in all cases. (And to answer the question that wasn't asked, since it is shorter than whilst and clearly understood by US as well as British English-speakers, I'd say stick with while every time.)

According to the Oxford English Dictionary, the differentiation between prophecy and prophesy as noun and verb, respectively, happened more or less arbitrarily; before circa 1700, both variants were used for both noun and verb each:

ME. a. OF. prophecier (1245 in Godef.), -phesier, -fecier, -ficier, f. prophecie, -fecie prophecy. The modern differentiation of prophesy vb. and prophecy n. was not established till after 1700, and has no etymological basis, prophesy being at first a mere spelling variant in both n. and vb.

The reason why this pair exists, while either c or s was chosen for both forms in other Greek words, is most probably confusion about which Greek word they were derived from. Normally, Greek -tia results in English -cy/cie, while Greek -sis results in English -sy/se/sis (both through French). This development is probably related to two facts:

• C and s have long been pronounced the same before y, so that they are easily confused. Vitaly has automatically compiled a list of words of Greek origin that have been found to occur with either spelling in major dictionaries; this illustrates how c and s were often interchangeable in the past. Note that most, if not all, of these words are now only spelled either -cy or -sy. Note also that one could no doubt find many more searching the Oxford English Dictionary for alternate spellings by hand.

acracy/acrasy, apostacy/apostasy, eustacy/eustasy, idiosyncracy/idiosyncrasy, isostacy/isostasy, prophecy/prophesy, syncracy/syncrasy, theocracy/theocrasy

• Most Latin and Greek words first came to us through French, in which -ti- was often softened to -ci-. The original Greek word was prophêteia, which would normally result in -c- in (Old) French; that is probably where prophecy came from. The -t- in Greek is there because the word is derived from prophêtes ("prophet").

But a more common way of forming a word with a similar meaning from the same verb stem (phê-) in Greek would be prophêsis (cf. haerêsis => heresy); from this, the French derivation would result in -sie, -se, or -sis (though -sis is rather modern, as it is the exact Latin transcription of the Greek word). Prophêsis did not exist in classical Greek, because their only word for prophecy and the action of prophesying was not derived directly from the verbal stem, but from the noun prophêtes as above. But ignorance of this fact is probably what led people to spell -s- sometimes.

Perhaps certain writers thought the s was closer to the Greek and preferred it, especially in the verb, because they though it was -t- in the Greek noun and -s- in the Greek verb (which is not true). Or perhaps the differentiation developed for even more arbitrary reasons.