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:
finedictionary.com which kind of doesn't fit in the context, and
thesaurus.com which perfectly fits in the context, but I couldn’t find this meaning in any of several online dictionaries.