This is an extract from 'The Man Who Knew Too Much' by Alexander Baron:
Corporal Turnbull was a young man, but he was not a man to be trifled with. He had come back from Dunkirk with all his equipment correct and accounted for and his kitten in his pocket.
What does the highlighted phrase mean? The only meaning I could find online was (and I'm not even sure of its validity) that it means 'hiding your anger'. That doesn't seem to be relevant in the stated context.
Best Answer
To have a kitten in (one's) pocket is not an idiom in English. The idea of repressed or controlled anger is only a guess.
Almost all the results from a Google search referenced that passage or instances when someone actually had a kitten in a pocket.
How seriously would you doubt the abilities of a man who was so capable that he came "back from Dunkirk with all his equipment correct and accounted for and his kitten (safe and calm) in his pocket."