A cracking sound.

(colloquial) The vertical ridge visible between a person's abdominal muscles, when highly toned.

crazy; demented

plural of crack-hemp

(obsolete) A person who deserves to be, or is likely to be, hanged.

plural of crack-rope

(obsolete) A person who deserves to be, or is likely to be, hanged; a gallows bird.

simple past tense and past participle of crack up

(informal) A sudden, unexpected and unintentional change in the pitch of one's voice when speaking or singing, which often affects boys during puberty.

An expert marksman with a gun.