1
general::
crack up
informal
to become mentally ill
• I think she's cracking up., crack (sb) up
[ M ] informal
to suddenly laugh a lot, or to make someone suddenly laugh a lot
• I took one look at her and cracked up.
• There's something about that guy's face that just cracks me up., crack down
to start dealing with bad or illegal behaviour in a more severe way
• The library is cracking down on people who lose their books.
Cambridge-Phrasal Verbs
2
general::
In addition to the idioms beginning with CRACK, Also see BY JOVE (CRACKY); FALL BETWEEN THE CRACKS; GET CRACKING; HARD NUT TO CRACK; HAVE A CRACK AT; MAKE A CRACK; NOT ALL IT'S CRACKED UP TO BE; PAPER OVER (THE CRACKS).
American Heritage Idioms
3
general::
verb break ADV. badly The dish had cracked quite badly. | slightly become mentally ill ADV. completely | finally The stresses of her job became too great and she finally cracked. | up He thought he'd never get through the ordeal without cracking up. PHRASES crack under the pressure/strain, show signs of cracking He is under a lot of pressure but is showing no signs of cracking. PHRASAL VERBS crack down ADV. hard PREP. on to crack down hard on crime, noun line on the surface of sth where it has broken ADJ. big, deep, huge, large, long, serious | short, small | fine, hairline, thin CRACK + VERB appear | develop, spread | run A fine crack ran up the wall. narrow opening ADJ. narrow, small, tiny | wide VERB + CRACK fill We filled the cracks in the plaster before hanging the wallpaper. CRACK + VERB appear Wide cracks appeared in the ground during the drought. | open up, widen PREP. ~ in There's a crack in the fence big enough to look through. PHRASES a crack of light a tiny crack of light under the door | open a door/window a crack Could you open the window just a crack? sudden loud sound ADJ. loud, sharp VERB + CRACK give, let out He gave a crack of laughter. | hear PREP. with a ~ The chandelier hit the floor with a crack. | ~ of PHRASES the crack of a whip, a crack of thunderSOUND
Oxford Collocations Dictionary