crack

Concise Oxford English Dictionary © 2008 Oxford University Press:
crack/krak/

▶noun

• 1 a narrow opening between two parts of something which has split or been broken.

• 2 a sudden sharp or explosive noise.

  ■ a sharp blow.

• 3 informal a joke or jibe.

• 4 (also craic) chiefly Irish enjoyable entertainment; a good time.

  ■ Scottish & N. English a conversation.

• 5 informal an attempt to do something.

• 6 (also crack cocaine) a potent hard crystalline form of cocaine broken into small pieces.

▶verb

• 1 break or cause to break with little or no separation of the parts.

• 2 give way under pressure or strain.

  ■ (crack up) informal suffer an emotional breakdown under pressure.

  ■ (crack up) informal burst into laughter.

• 3 make or cause to make a sudden sharp or explosive sound.

  ■ hit hard.

• 4 (of a person's voice) suddenly change in pitch, especially through strain.

• 5 (crack down on) informal take severe measures against.

• 6 informal solve, interpret, or decipher.

  ■ break into (a safe).

• 7 (crack on) Brit. informal proceed or progress quickly.

• 8 tell (a joke).

• 9 decompose (hydrocarbons) by heat and pressure to produce lighter hydrocarbons.

▶adjective very good or skilful: a crack shot.

– phrases
be cracked up to be [with neg.] informal be asserted to be: acting is not as glamorous as it's cracked up to be.
crack a crib archaic, informal break into a house.
crack of dawn daybreak.
crack of doom a thunder peal announcing the Day of Judgement.
crack of the whip Brit. informal a chance to try or participate in something.
get cracking informal act quickly and energetically.

– derivatives
cracky adjective.

– origin OE cracian ‘make an explosive noise’; of Gmc origin.