What's the origin of the phrase "shot away" as meaning a person is behaving in an unhinged manner?
Green's Dictionary of Slang offers this entry under the headword shot.
- (orig. Aus., also all shot, shot at, shot away, shot through) of a person, exhausted or in bad shape.
A variant of this is first attested in a quote by F. Scott Fitzgerald in 1920:
F.S. Fitzgerald ‘May Day’ in Bodley Head Scott Fitzgerald V (1963) 144: You certainly look all shot.
Not until 2000 does GDoS offer an attestation using the form "shot away," in the context of drugs:
2000 [UK] Indep. on Sun. Rev. 21 May 6: He was so shot away on pills and alcohol that he drove his car off the road.
It's tempting to attribute all variations of this to the verb shoot up meaning to intravenously inject a drug, but it's worth noting that variations referring to alcohol intoxication appear to be the oldest, such as this 1838 attestation of "half shot," meaning "slightly tipsy, or partially drunk."
1838 [US] J.C. Neal Charcoal Sketches (1865) 13: If your tongue wasn’t so thick, I’d say you must mosey: but moseying is only to be done when a gemman’s half shot.
It is worth noting that there are many citations of shot meaning drunk that predate the phrase shoot up meaning to inject a drug, which is first cited in 1905 by GDoS:
1905 [US] F. Hutchison Philosophy of Johnny the Gent 70: ‘[I]f a doctor hadn’t [...] shot him full o’ dope he’d ’a had the snakes on the square’.
It's possible the earlier references to being shot meaning drunk are figurative extensions of something that has been "shot by a gun."
For what it's worth, the OED indicates that a "shot" has been used to refer to a small portion of something since the 17th century -- this is likely the origin of the word shot in shotgun and shot-glass, as opposed to the inverse, which might seem more intuitive.
shot etymology etymonline
Figurative sense "ruined, worn out" is from 1833.
a possible reference to a person's demeanor.