there's no telling
Something is unknown because it cannot be predicted. He acts so erratically that there's no telling what he will do next.
there's no telling
It's impossible to determine, as in There's no telling how many children will come down with measles, or There's no telling what will happen in the next episode of that soap opera. This idiom uses telling in the sense of "reckoning," a usage dating from the late 1300s.
there's no telling
it's impossible to know what has happened or will happen.