The etymology of "done for"

Solution 1:

Etymonline is a good resource for etymologies. They don't have all idioms, but they do have quite a few of the most common:

done

past participle of do (v.); from Old English past participle gedon (a vestige of the prefix is in ado). As a past-participle adjective meaning "completed, finished, performed, accomplished" from early 15c. As a word of acceptance of a deal or wager, 1590s.

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To be done in "exhausted" is by 1917. Slang done for "doomed" is by 1803 (colloquial do for "ruin, damage" is from 1740). To have done it "to have been very foolish, made a mess of things" is from 1837.

If you want the fuller story, though, check out the OED if you have access:

2. intransitive. colloquial. To ruin, damage, or injure fatally; to destroy, wear out entirely. Now frequently in prepositional passive. 1475 Sidrak & Bokkus (Laud) (1998) I. l. 5902 By thre skyllis may hit come to Why hit [sc. a stillborn child] is don for [Lansd. fordone] alsoo.
   1740 Sessions' Paper 9 July 190/2 D–mn you, I'll do for you.
   1752 H. Fielding Amelia II. vi. iv. 215 He said he would do for him..and other wicked bad Words.
   1803 Ld. Nelson 28 Dec. in Dispatches & Lett. (1845) V. 334 The Kent is almost done for, and she is going to Malta.
   1811 J. Austen Sense & Sensibility III. v. 104 He has done for himself completely!—shut himself out for ever from all decent society.
   1817 J. Austen Persuasion (1818) IV. xi. 279 Give Anne your arm... She is rather done for this morning.
   1843 J. S. Robb Streaks Squatter Life 128 They found Sam holding the straw figure in his arms, and looking in a state of stupor at the horse; he thought his master was ‘done for’.
   1876 C. D. Warner Winter on Nile i. 18 The railway up the Nile had practically ‘done for’ that historic stream.
   1918 S. Sassoon General in Counter-attack & Other Poems 26 He did for them both by his plan of attack.
   1991 D. Dabydeen Intended (1992) 188 They come in and try to steal, if you don't keep a sharp eye on them you're done for.

The first citation I think is the key here. "Done for" in Middle English could be written "fordone." The prefix "for-" has ruinous connotations:

for-

prefix usually meaning "away, opposite, completely," from Old English for-, indicating loss or destruction, but in other cases completion, and used as well with intensive or pejorative force, from Proto-Germanic *fur "before, in" (source also of Old Norse for-, Swedish för-, Dutch ver-, Old High German fir-, German ver-); from PIE *pr-, from root *per- (1) "forward," hence "in front of, before, toward, near, against."

In verbs the prefix denotes (a) intensive or completive action or process, or (b) action that miscarries, turns out for the worse, results in failure, or produces adverse or opposite results. In many verbs the prefix exhibits both meanings, and the verbs frequently have secondary and figurative meanings or are synonymous with the simplex. [Middle English Compendium]

Probably originally in Germanic with a sense of "forward, forth," but it spun out complex sense developments in the historical languages. Disused as a word-forming element in Modern English. Ultimately from the same root as fore (adv.). From its use in participles it came to be an intensive prefix of adjectives in Middle English (for example Chaucer's forblak "exceedingly black"), but all these now seem to be obsolete.

So "done for" means "brought to the point of ruin." You can compare this to "finished," which often means "ruined," as in "He got the money? Oh, no, I'm finished!"

Interestingly, there might be a connection with a secondary meaning of "done" meaning "arrested" or "convicted."

From the OED:

1784 Sessions' Paper Jan. 221/1 He stepped on one side of me and said, ‘You have not done me yet.’ I immediately pursued him.
1819 J. H. Vaux New Vocab. Flash Lang. in Memoirs II. 168 Done, convicted; as, he was done for a crack, he was convicted of house-breaking.
1936 ‘G. Ingram’ Muffled Man vi. 91 Blow me if one of your tribe [sc. policemen] don't go and do me, and I get found a quid.
1963 Guardian 23 Feb. 4/4 ‘This is a murder charge. There is no certainty that you will be done for murder.’..He did not say that Kelly would only be ‘done’ for robbery and not murder.
1968 ‘R. Simons’ Death on Display iii. 44 I'm goin' straight. Last time I was done was two years ago, and I ain't been tapped on the shoulder since.
1995 Daily Mail 2 Jan. 3/1 He intends you to be done for drink-driving.