Unfortunately, this useful and effective insult for a stupid person or a blockhead has rather dropped out of use in these mealy-mouthed times.
One of the last excursions for it that I can find is in the classic W C Fields film The Bank Dick of 1940, in which the word occurs in a variant form in the line: “Surely, don’t be a luddie-duddie, don’t be a moon-calf, don’t be a jabbernow, you’re not those, are you?” Before that, it turns up in one of the novels of Hall Caine in 1890, but even by then it seems to have been rather rare.
Most sightings today are in books of weird or unusual words, but I was pleased to to find it not long ago in Henry Mitchell on Gardening, published in 1999: “When I discovered I could grow it here — I like to say any jobbernowl can — I was as pleased as a dog with two tails.”
It’s from old French jobard, from jobe, silly. That word was then added to noll, the top or crown of the head, the noddle. The first sense was of a blocky or stupid-looking head, but was soon extended to refer to the quality of the mind within.
Jobbernowlism is the condition or state of being a jobbernowl, or an act or remark that is especially stupid. I’ve also found an instance of an even greater rarity, jobbnowlry, which appeared in 1985 in Gods of Riverworld by Philip José Farmer: “‘Sheer jobbernowlry, darkest superstition,’ Burton had said.”
Careful how you use it: the recipient might be a subscriber!