can't hold water 

When you confide in someone and they immediately tell someone else when you told them not to

Cant keep a secret

I told Alicia furtively about my vaginal disease and the next day I found out that she told her boyfriend, and the cheer team! Black people can't hold water for nothing!

can't hold water by Cindyloohoo March 5, 2014

you can't hold water

Means you can't keep any secrets period

OMG I told Alyssa to keep the engagement a secret but she blew the surprise. I told her "you can't hold water damn"

you can't hold water by Ladycola September 3, 2016