The Jacobite Christians of Egypt, who have for eleven centuries been in possession of the patriarchal chair of Alexandria. The word is probably derived from Coptos, the metropolis of the Thebaid. These Christians conduct their worship in a dead language called “Coptic” (language of the Copts).
“The Copts [of Egypt] circumcise, confess to their priests, and abstain from swine's flesh. They are Jacobites in their creed.” —S. Olin; Travels in Egypt (vol. i. chap viii. p. 102).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894