From Longman Dictionary of Contemporary Englishcry/sing etc your heart outcry/sing etc your heart outif you cry, sing etc your heart out, you do it with all your energy or emotion He found me crying my heart out and was so kind. → eat your heart out at eat(4), → pour your heart out at pour → heart
Examples from the Corpus
cry/sing etc your heart out• Anyway, then she just sprawled on the floor and cried her heart out.• After this last furlough we paid a last sad visit to the school and found a small girl crying her heart out.• March 7: I cried my heart out last night after seeing the movie High Tide.• For the first time since I cried my heart out in Puerto Rico - I was crying.• We would march along in step, doing eighty-eight paces to the minute, singing our hearts out.• She cried her heart out, all because of an unruly trouble-making, black-hearted child who was ripping her apart. 