prophecy

First recorded in 1175–1225; Middle English prophecie, from Old French, from Late Latin prophētīa, from Greek prophēteía; see prophet, -y³

The French-derived noun prophecy and the related verb prophesy have a unique history. Before English spelling became relatively stabilized, they were both spelled many different ways—some with a c, some with an s, and even, at least in the case of the noun, some with a t (as in the corresponding modern French form prophétie ). But in the 18th century, the great diversity of spellings for these words settled down, with the c form becoming standard for the noun and the s form for the verb. At some point the pronunciation of the verb was also distinguished from that of the noun, so that instead of rhyming with see, like the noun, the verb rhymed with sigh —perhaps by analogy with the many verbs ending in -fy ( testify, stupefy, etc.).
Considering the close relationship between the words, it is not surprising that they are easily confused; in particular, it is not unusual to see the noun written with an s, just as was often done before the 18th century. We may even prophesy that, over time, the form will once again become a completely acceptable spelling for the noun. But until then, careful writers and speakers maintain the conventional and long-established distinction between the two words in both spelling and pronunciation. Similarly, the verb "prophesize" (or "prophecize")—resulting from confusion between prophesy and verbs ending in -ize like proselytize and prioritize —is regarded as nonstandard. When you make a prophecy (sounds like see ), you are prophesying (sounds like sighing ).

C13: ultimately from Greek prophētēs prophet