Several meanings. Most commonly refered to as one who is doing stupid things, as in a person who is so out of it, they are trying to smack crack into their skin directly. Also, can be used as a term of endearment, or torment depending on the person.
don't taunt the bullwith thatred cape, you crack smacker.
or,
that was so sweet of you to buy me those flowers you are such a crack smacker
a girl than performs annilingus "rimming" while in the process of giving a comprehensive blowjob that includes the balls (in other words, sticks her tongue around and in the anus after licking and sucking the balls, and then back again, while always keeping the cock in play. The drill:cock, balls, ass--wherever the mouth isn't, the hands are