In a 1951 Humphrey Bogart film (The Enforcer; Murder Inc in the UK), both the DA and the police are befuddled by the terms 'hit' (noun form, meaning 'a person designated to be killed') and 'contract' (as in 'put out a contract on'). Indeed, much is made in the early part of the film about their not understanding what these word refer to. And my Random House Unabridged gives the date of origin for 'hit man' as 1965-70. Are all these terms really that recent?