"Jago"

In Robert Crisp's book, he described how they would use the radio callsign "Jago" (I may be misunderstanding its use). They would say "Jago Jago". I wondered at the origin of the word. From Ian Paterson's site, I noticed the mention of Lt-Col R. K. Jago, apparently commander of the 3rd County of London Yeomanry. Ian Paterson mentions him in the section about the Crusader Battle, and the fighting on 19 November 1941 near Bir El Gubi. Ian says that the 22nd Armoured Brigade, of which the 3rd CLY was part, lost about 40 Crusaders to anti-tank gun fire. As we have seen, the 50mm PAK38, suitably emplaced, could be quite deadly to British tanks. The low silhouette made them easy to hide. This doesn't explain the use of "Jago" by the 3rd RTR, but at least provides a possible origin for the word.