On Wed, 28 Jun 2006 Paul Terry wrote:

The apostrophe (to indicate elision) was used in French and in Italian
before it appeared in English (from soon after 1500), and is still used
in both languages (d'Avignon, d'Italia, etc) for the same purpose.

It was used in the same way in English ("Th'expense of spirit in a waste
of shame"). But one of the most common examples was to show the omitted
final e in the genitive singular of Old English (which ends with -es in
the majority of nouns) - thus Kinges became King's and childes became
child's. And from this the apostrophe-s ('s) came to be used for the
genitive (possessive) form of most nouns, thus representing the spoken
form of the language more faithfully than the Old English form.

I always thought it was from omitting hi in King his, leading to King's.

--
Thoss