I imagine it's from Luke 1:26, where the angel Gabriel greeted Mary with "chairo", which is greetings. Not "hail" like we think of hailing Hitler after WW2. It'd be analogous to saying "howdy".
Is there more in scripture about talking to Mary in our prayers, than just an angel giving her a message and beginning it with "greetings"? Don't get me wrong. That was truly a big event and Mary is one of our Bible heroes. But that's a far cry from saying we should pray to her.
No, it’s not howdy. You don’t hail your average person, much less announce that they are “full of grace”. That whole sentence is full of profound meaning. That is why she is amazed and not terrified like everyone else who is greeting by an angel.