To cry your eyes out is a figurative expression that means to cry uncontrollably, a lot or/and for a long time until your eyes become itchy.
We use the expression to describe someone who’s excessively unhappy.
Let’s see some examples :
– When my grandfather died, I cried my eyes out.
– Poor boy! He’s still in his room crying his eyes out since she broke up with him.
– I can’t just sit here watching you crying your eyes out. You have to move on, man.