In a state of agitation or excitement; whetting or sharpening (archaic).
From 'a-' plus 'whet' (to sharpen). Originally meant sharpening or stimulating, then came to mean in a state of stimulation or agitation as the metaphorical sense developed.
The word 'whet' (as in 'whet your appetite') comes from Old English and means to sharpen—but it's also used metaphorically to mean 'stimulate or excite,' which is why you see it in appetizing contexts.
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