laugh, shout, scream, etc. your head off idiom informal
C2 持续地大笑／喊叫／尖叫（等等）
to laugh, shout, scream, etc. very noisily and for a long time
There I was lying face down on the pavement and you two were laughing your heads off! 我就在那儿脸朝下趴在人行道上，而你们两个却在那儿大笑不止！