A useless piece of equipment, usually military, used to refer to a mule-drawn freight wagon which had been pieced together from dicarded or substandard parts, and subject to frequent breakdowns. Jackwagons typically were good for only one or two uses, then abandoned or discarded along roadsides and in ditches, and were often re-cannibalized to create new jackwagons.
"Maybe we should trot on over to mambypamby land and get you som confidence - you jackwagon!" Geico commercial
by LTC's Daughter-in-law December 27, 2010