The story
A patronymic surname that is derived from the male given name John + -kin (which forms diminutive )+ -s (denoting "son of") hence meaning "son of little John". It was originally an offshoot of the male medieval name Jenkin/Jankin. The name was brought from the crusaders; it originated in Cornwall but became popular in England and Wales.
Etymology from Wiktionary, CC BY-SA 4.0.