“another fun non-latin word this one is, attested from c13c, making it comfortably middle english” {late middle english, to be exact}

the a- prefix here is actually not the latin based a- = negation, but rather the German er- / ar- reduced form, like in words “abide, arise, awake, ashamed”.

It’s that sense because the origin of the word is a- gast, “-gast” being taken from the middle english verb ‘agasten’, constructed from a- (intensifying/momentary) -gasten (to frighten), agast being the past participle conjugatoin, meaning “to have momentarily frightened in the past”

Gasten is evolved from old english gæstan “to frighten”, which itself is constructed from old english gæst “Spirit, ghost” (same word we get ghastly/ghostly/ghost from)

the introductoin of the -gh- in place of the original -g- is “probably” from flemish. “darned flemish” {I’m partial the dutch myself, as you can see :P}

Aghast became a generalized adjective in middle modern english, circa 17c