Origins and Spelling of the Name
I have heard many plausible explanations. One is that we’re all descended from illegitimate children of Julius Caesar, and that the name is a corruption of Julianus. I like that one. Nobody knows.
The earliest record I’ve seen is a sale of land in Brentwood, Essex from a Farmer Jillings to Sir Anthony Browne, for him to build Brentwood School. That was in the 1550s, but sadly I’ve not found anything else about him.
Most of the earliest records have differing variations on the name. Julians, Juliens, Jillens, Jillions, Jillons, Jellings (often a misreading of the written “Jillings”), as well as Jillings. The earliest Gillings that I have found (so far) is around 1720.
The great majority nowadays spell the name Jillings or Gillings – there is glorious confusion about which is which. Both spellings occur regularly in the same family. Many of these people were illiterate, so the spelling depended entirely on how the Vicar thought it should be when he baptised/married/buried them or how the Census Recorder decided. There are plenty of examples of the same family at the same address being spelled both ways over the years.
The willing and hard-working volunteers who have been transcribing census and other records also have trouble reading the handwriting in some of the older documents, and variants such as Sillings, Fillings, Billings and more are often to be seen. Returning to the original document usually reveals Jillings or Gillings.