How interesting, Eddie. I, too, am surprised that these terms are so recent. I guess I'd subconsciously planted them firmly in the gangster days of the 1920s.

My slang and etymological dictionaries offer no help, perhaps because the meanings are still fairly close to the original meanings of the terms.

To "hit" includes to strike, which is the point of and the method of execution (*heh*) of a hit.

I agree with Maverick that a "contract" probably is a contraction of "contract killer" - a killer who is under contract, hired to do the deed for an agreed price.