The Latin name Johannes eventually gave us Jean in French and John in English. In medieval times Johns were often more familiarly known as Jan or Jen. This form then had -kin added to it to give the 16th century Jenkyn, Jankyn and similar. Before becoming a hereditary surname the suffix -kin was first used to denote the younger of two family members with the same or similar names.
As with Hockin, there was a tendency in the 1600s for a -g to be added to the name to make Jenking. But unlike Hocking, this process then reversed itself (apart from at St Just in Roseland.) Instead, an -s had been appended to 17 per cent of the 508 Jenkins who were household heads in 1861.
Jenkin is a name that was in the early 1500s associated very clearly more with Cornish-speaking mid and west Cornwall. The 18th century industrialisation of west Cornwall then reinforced this distribution. It encouraged earlier marriages and a birth rate that was one of the highest in Europe for a time, consolidating this surname’s position in the top 20 Cornish names. By 1861 most Jenkin households were found in Kerrier and Penwith in the far west.
Cornish studies resources 
My great grand father was a Jenking. Born late 1800s. My parents knew him as an elderly man, and his daughter, a Jenking, my great grandmother, I knew in Truro, [due to WWI, she married and lost husbands: Penhaligon, Tregaskis, Dart], also lived into her 90s, (born in 1901, Truro area? Possibly Roseland, as there were latterly connections to the Laity[s] and Curnow[s] from Flushing).
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Missed a great on great great grandfather Jenkings!
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Commonly,
Cornwall is Jenkin, Wales is Jenkins
Cornwall is Johns, Wales is John.
😂 That’s the quirkiness of surnames. That’s why we find the subject intriguing.
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Meaning of -kin has been argued. It seems (across England) it was not just a diminutive but meant eldest son or heir.
There were about 30 Jenkins as a forename in the 1522 Military Survey, a few Jankyns too – and some triple-banger patronymics (son and grandson of) “Jenkin John Pascowe”, Jenkyn John Hycke” and “Jenkyn John Hoskyn”. Also, by the way, “Hockyn Enowre” and several other Hockyns as forename .
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What’s the source for the idea that -kin was the eldest? That theory directly contrdicts the ODFN.
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