[1] Jen/Jean (pronounced "Jon") being a diminutive of Jehan/Jehannes* (John/Johan*) followed by kin/ken meaning little creating Jenkin or Jenken.
[2] The name "Jenkin" or "Jenken" first use in England is seen as early as 1086 as a diminutive of the English form of John.
It first was found in Monmouthshire in the Domesday Book of 1086 and some say earlier than 1066 and the Norman Conquest of England.
[5] Its common English use, eventually becoming a surname, may have come as a generic and now obsolete nickname as the "Little Johns".
This may have been a 12th Century reference to the Welsh and Cornish people because of their relative smaller stature or more likely as a derogatory for the subjects or illegitimate offspring of King John of England, Earl of Cornwall and Gloucester (1166–1216).