not able to hold it (in)
No longer able to prevent oneself from urinating. I tried to sit through the rest of the movie, but I wasn't hold it anymore and raced out of the theater to use the bathroom. I was worried our son wouldn't be able to hold it until we got home.
not able to wait
to have to go to the bathroom urgently. (Also more broadly literal.) Mom, I can't wait. Driver, stop the bus! My little boy can't wait.