• Crack down: Enforce more stringently or more thoroughly
• Crack down on: Enforce laws or punish (something) more vigilantly
• Crack on: Continue at a (normally uninteresting) task
• Crack on: Continue apace
• Crack up: Laugh heartily
• Crack up: Cause to laugh heartily
• Crack up: Become insane; to suffer a mental break down
• Crack up: Cry up; to extol
• Crack out: Produce in large volumes mechanically or as if by machine
