From where did Mary get the attribution of "Full of Grace"? The Catholic prayer referred to as a "Hail Mary" ascribes the phrase "full of grace" to Mary.  The Scriptures describe both Jesus and Stephen as being "full of grace".  In Greek, this is "πλήρης χάριτος".


  And the Word became flesh and dwelt among us, and we have seen his
  glory, glory as of the only Son from the Father, full of grace and
  truth.  John 1:14 ESV
  
  And Stephen, full of grace and power, was doing great wonders and
  signs among the people.  Acts 6:8 ESV


However, there doesn't seem to be any place where the Bible describes Mary in this way.  The only place that is close is in Luke 1:28 where the angel greets Mary and indicates that she has been given grace, often translated as "highly favored" (κεχαριτωμένη in Greek).

So, certainly Jesus was "full of grace" and Stephen was identified as being "full of grace".  Yet, why does Mary also receive this ascription?
"Hail, full of grace, the Lord is with you," is an appropriate translation of the Greek at Luke 1:28, which reads 


  και εισελθων ο αγγελος προς αυτην ειπεν χαιρε κεχαριτωμενη ο κυριος μετα σου ευλογημενη συ εν γυναιξιν.


(Scrivener 1894 Greek NT)

The key phrase in that passage is "Chaîre, kecharitōménē, ho Kýrios metà soû." (NOTE: I am transliterating the Greek to its English equivalent.) Kecharitōménē is a hapax legomenon (literally a word or phrase that is "expressed once" in all known Greek literature).

Kecharitōménē is the verb χαριτόω (charitóō) rendered in the perfect participle passive. In researching the word, I learned that the perfect participle passive describes a state that exists at the time coincident with that of the leading verb as a result of action completed prior to the time of the main verb. See, http://www.ntgreek.net/lesson34.htm. Thus, one could translate "Chaîre, kecharitōménē, ho Kýrios metà soû" in English as "Rejoice, you who have been, and are, filled with grace, the Lord is with you." "Grace" here would refer to God's grace of salvation.

This word is important in understanding Catholic doctrine and dogma related to Mary. Reading this passage in the four senses of Scripture [literal, allegorical, moral (aka tropological), and anagogical] gives some good insights into the thinking leading to the Catholic Marian doctrines.