I am quoting from the Memoirs of Sherlock Holmes, The Reigate Squires by Arthur Conan Doyle: "'There are some very singular points here,'said Holmes, smiling. 'Is it not extraordinary that a burglar - and a burglar who had had some previous experience- should deleberately break into a house at a time when he could see from the lights that two of the family were still afoot'. 'He must have been a cool hand'". I found the following sources:
It appears to be a dated, originally mainly a BrE expression:
a cool, calm, controlled and competent individual:
- 1840 [UK] R. Barham ‘Black Mousquetaire’ in Ingoldsby Legends II (1866) 210: A fact which has stamp’d him a rather ‘Cool hand’.
- 1855 [UK] T. Taylor Still Waters Run Deep II ii: I’m a cool hand, I flatter myself, but, by Jove, she nearly threw me off my balance last night.
- 1936 [UK] N. Marsh Death in Ecstasy 143: He’s a very cool hand is monsieur.
- 1965 [US] D. Pearce [title] Cool Hand Luke.
A "cool hand" would be someone who can remain "cool" under pressure. Meaning under control. Cool -- the opposite of hot as in hot tempered. The "cool hand" can keep their hands from shaking even with the extra threat of discovery by an awake home owner. Sherlock believes that would be a very, very good burglar who could do that.