Jenkin, of Franconian origin, is translated in English as "Little John" or more literally "John the little."
Jen/Jean (pronounced "Jon") being a diminutive of Jehan/Jehannes* (John/Johan*) followed by kin/ken meaning little creating Jenkin or Jenken. *(Referred to as Johannes in the Latin and Germanic referring to the Bible name John.) 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 was often translated from the Dutch/French as "John the younger" or seen as "John Jenken." The non-diminutive Jehan/Jehannes (pronounced "Jo-han/Jo-han-nes") was also translated into English as John. When Jen/Jean is present, usually given to a younger child, Jehan/Jehannes is listed as "John the elder" but, never translated as "Big John."
Confusion can arise when the sire is listed as John, a son is John (the elder) and another son is John (the younger). Today, in English the term John, Senior is used for the father, while the names of John can use Junior or numeric designation (i.e. "II"). "Jon" the phonetic of John is sometimes seen but only in males as is the younger male nickname of "Johnny." The name "Jean" once pronounced "Jon" in English and once a male name has become since the 16th century a female name in English from the French Jeanne.
Jenkin is a surname variant of Jenkins commonly seen in Cornish and in English (mainly Devon) ancestry. Its translation is "Little John" or more literally "John the little." It first was found in Monmouthshire in the Domesday Book of 1086 and some say earlier than 1066 and the Norman Conquest of England.