Agape means with the mouth hanging wide open in astonishment. The jaw has dropped and stayed there.
It follows the noun rather than preceding it, so standing there with his mouth agape, and it belongs to the small family of a- adjectives that behave that way alongside asleep and ablaze. It is formal and slightly literary. There is a completely separate word spelled the same and pronounced differently, since agape in three syllables is a Greek word for selfless love used in theology.
1adjectiveagapeWith the mouth wide open in astonishment.
“He stood there with his mouth agape.”
Also accepted in games: open-mouthed
Old Norse
gapa — to gape
Middle English
gapen
English
gape
English, today
agape
2009 From a + gape. First known use by John Milton in Paradise Lost (1667). gape comes from Middle English gapen, from Old Norse gapa (“to gape”) (compare Swedish gapa, Danish gabe), from Proto-Germanic gapōną (descendants Middle English geapen, Dutch gapen, German gaffen), perhaps from Proto-Indo-European ǵʰeh₂-. Cognates include Russian зяпа. a doublet of gap.
Etymology from Wiktionary, CC BY-SA