Kecharitomene

In Luke 1:28, the Archangel Gabriel greets the Blessed Virgin Mary with the words, "Hail, Full of Grace."

The Greek for "Full of Grace" (Gratia Plena in the Vulgate) is "kecharitomene" which comes from the verb "to endow." The word kecharitomene is a perfect passive participle. It refers to an event completed in the past, but with continued relevance to the present.

It is only used elsewhere one other time in the New Testament, and in that case it is not a perfect passive participle.

In the greeting from Luke 1:28, the Archangel is giving Mary a new name. She is now "Full of Grace", who was once known as Mary.

Putting all this together the Archangel is saying to the Virgin, "Hail, You who have been completely, perfectly, and enduringly endowed with grace."

This is why the verse is the seed of the Immaculate Conception.

Origen, in his commentary on this passage says:
"Because the angel greeted Mary with new expressions, which I have never encountered elsewhere in the Scriptures, it is necessary to comment on this. I do not, in fact, recall having read in any other place in the Sacred Scriptures these words: Rejoice, O Full of Grace. Neither of these expressions is ever addressed to a man: such a special greeting was reserved only for Mary."