I was curious about the name and did some research on wikipedia (I know, I know ...)
It appears that as a surname it has a fairly long history. There are Anglo-Saxon roots to it, though folk etymology has it also going back to forms of St Denis, and thus (Norman) French. One of the most notable Sidneys in history is perhaps one of my favourite poets, Sir Philip Sidney, one of the first English writers of sonnets. From Elizabethan until early Georgian times, the Earls of Leicester were Sidneys. In the 18th century, a Townshend was created Baron Sydney and later elevated to Viscount Sydney; the Australian city is named after him. This Barony/Viscountcy was still extant in JA's times.
The use of Sidney as a Christian name seems to go back to one Algernon Sidney, an English republican whom Charles II had executed for treason in 1683. It seems that the name was more popular in post-Independence USA than in Britain for precisely those Republican, anti-monarchical leanings which I gather were more en vogue on the other side of the pond ;-)
I don't know if that has any importance for our Sidney Parker, but I figured I might just as well share my learnings.
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