Strange English mnemonic: 'S' before 'N', except after the 15th century

As Third Idiot noted, some of these “s before n” come from a Old French, along with a lot of cases of “s before other consonants”. Most of them came from Latin, either directly (sperare gave espoir [hope], hospes, hospit- gave hospital, testa gave teste [head]), others by contraction (asinus gave asne [donkey]) or deformation (fraxinus gave fresne [ash tree]). As an example of a word with close history to the one you're enquiring about, puisne (a legal term) comes from Old French puisné, from Latin roots post [after] and natus [born]. The French word is now written puîné, and I'll describe this process below.

There was later a movement of suppression of the s before many other consonants, starting in the 12th century and going all the way to the 17th. The s was dropped, but remained in pronunciation and was marked with a circumflex accent: asne became âne (pronounced /ɑn/), with the accent a sign that it is not to be pronounced /an/. Other examples above: hospital became hôpital, teste became tête, fresne became frêne, etc. The process wasn't exhaustive, as words with less usage (or scientific words) kept their s: e.g. the adjective hospitalier (cf. the noun hospital).

While this has been a long digression into French, it seems to confirm that people with knowledge of Old French at the time would be inclined to include s before other consonants, though I cannot figure out why n is singled out in the paragraph you quote.

Edit: NateMPLS cites rein, which I hadn't thought about. It comes from Old French rene (Modern French would be rêne), from earlier French resne, from Latin retina. It doesn't have the s in English because it was borrowed from French after the s was dropped.

Words that may have arised:

See the entry for "Rein" (Google books, Imperial Dictionary of the English Language)