This is an old British name of which the origins are pre Roman Conquest 55 to 410 A.d.,
although the surname is like all surnames, post Norman (after 1066).
It derives from "Scyd", (a hillside) and presumably once was prefixed with hill, bank, cliff etc.. It does in fact mean one who was resident at a place on a Slope, or originated from a village called "Syde or Side" as in Syde in Gloucester.
The pluralality of the name is either dialectal or implies "The son of Side" a form of patronymic. The name recordings include:
John Sides, a witness at Stoke-upon-Trent on August 7th 1796,
whilst earlier in 1620 one John Sydes (or Sides) married Jane Stearens at St. Benets,
London on January 15th.
The first recorded spelling of the family name is shown to be that of Herbert de Side, which was dated 1221, in the Assize Rolls of Gloucestershire, during the reign of King Henry 111, known as "The Frenchman", 1216 - 1272.
Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax.
Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.