Why is the ninth month called September?
"split, opening," 14c., from crack (v.). Meaning "try, attempt" first attested 1836, probably a hunting metaphor, from slang sense of "fire a gun." Meaning "rock cocaine" is first attested 1985. The superstition that it is bad luck to step on sidewalk cracks has been traced to c.1890. Adjectival meaning in "top-notch, superior" is slang from 1793 (e.g. a crack shot).
[all senses are ultimately echoic; narcotics sense fr the sound of breaking crystals or the cracking sound the crystals make when smoked]