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.