The term comes from the fact that there is a lot of information to be learned in a very short amount of time. The same expression is often heard at the airlines. I personally can’t stand the expression, it implies that something is impossible to do when in fact it is very possible. I have gone through airline training on four different jets, I have heard that expression every time, yet somehow managed to pass every checkride. Same thing with ATP, I heard the expression the whole way through, yet did well in the program and passed everything.
For some reason it is an expression that has taken hold, but it is not representative of what really happens in airline (or ATP) training. Sure there is a lot of information to digest, but it is doled out in appropriate sized amounts in a manner that has worked for decades. I personally think we need a new expression.