- 02.114 It would have made only a tiny pocket-knife for a troll,
The key feature, as OED explains is the foldability of blades to make the knife suitable for pocket carry. The examples there include the hyphen.
“pocket knife, n.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/146413. Accessed 19 September 2017.