Hello. As you can see, my surname is also Jehu.
My father was Ivor S. Jehu, born St.Andrews, Scotland in 1908, and his father was Thomas Jehu, Regius Professor of Geology at the University of Edinburgh, b. Llanfair Caereinion, near Welshpool, Montgomeryshire (now Powys), Wales in 1871, and his first language was Welsh. His father, John Jehu, was a corn merchant from the same village.
I have heard that all Jehus, at least of Welsh origin (there are others in Belgium and the department of Vendee in France), are descended from one Thomas Jehu of Llanfair Caereinion living at the beginning of the 17th Century.
In or about 1605, Kings James I of England ( VI of Scotland) ordered that all Welsh people should have surnames as in England, instead of the traditional patronymics. The latter are still used in Iceland, where if your father's given name is Magnus, for example, then you are called Magnusson as a surname, or Magnusdottir, if you are a girl.
Similarly a Welshman whose father was called John or Sion in its Welsh version would have as a patronymic "ap Sion". This became Upjohn, or in its anglicized version Jones, when permanent surnames were forced on the Welsh population. Similarly, we have names such as Powell (ap Hywel), Pritchard (ap Richard), Price (ap Rhys) etc.
The ancestor in question, Thomas Jehu, would originally have been called Thomas ap Jehu, because his father's given name was Jehu. The latter was plainly named after the biblical king who was responsible for the demise of Jezebel, and given that during the latter part of the 16th century the Reformation was in full swing, this kind name was not unusual.
All this was told to me in a letter, which I have lost, from one Leo Derrick Jehu, some 30 - 35 years ago, and the above information is from memory and may contain some errors, but I believe to be substantially true.
More detailed information could be obtained from my sister, Myfanwy Jehu, at firstname.lastname@example.org
Patrick (Reay) Jehu