agape (n.)

c. 1600, from Greek agapē "brotherly love, charity," in Ecclesiastical use "the love of God for man and man for God," a late and mostly Christian formation from the verb agapan "greet with affection, receive with friendship; to like, love," which is of unknown origin. Sometimes explained as *aga-pa- "to protect greatly," with intensifying prefix aga-. "The Christian use may have been influenced by Hebr. 'ahaba 'love'" [Beekes].

Agape, in plural, was used by early Christians for their "love feast," a communal meal held in connection with the Lord's Supper. "The loss of their original character and the growth of abuses led to the prohibition of them in church buildings, and in the fourth century to their separation from the Lord's supper and their gradual discontinuance" [Century Dictionary]. In modern use, often in simpler sense of "Christian love" (1856, frequently opposed to eros as "carnal or sensual love").

"with the mouth wide open" (as in wonder), 1660s, from a- (1) + gape (v.).