English from Latin "nobleman" or "patrician".
In Scotland the two names Patrick and Peter are generally fully interchangeable, possibly due to the similarity to Peter of the Gaelic Pàtair which is Anglicized as Patrick. Although not strictly synonyms as they come from different original sources, the interchangeability is so common across Scotland, and known in the site author's researches [RCH], that we classify them as such.
A correspondent [AW2] has noted the same usage for a man born in Galway, Ireland in 1835:
"I have a great grandfather who was referred to as both Peter and Patrick,
depending on the document viewed. He signed his naturalization papers as
Patrick though, and seemed to prefer that name."
Another correspondent [RD] has noted the use of the spelling Petrick in Belhelvie parish in Aberdeenshire and used as a common name for both Peter and Patrick.
The surname Patterson may be derived from either Patrick or Peter .
In old documents in Latin, the form Patricus may be used for Patrick, with appropriate case endings.