Agapetos is foundationally based on the word "Beloved".
"And lo, a voice from heaven saying, This is my beloved Son, in whom I am well pleased" (Matt. 3:17).
ἀγαπητός agapētós, ag-ap-ay-tos'; from G25; beloved:—(dearly, well) beloved, dear.
James Strong. The New Strong’s Exhaustive Concordance of the Bible. (Tennessee: Thomas Nelson, 1995), 1.