Yours, Etc.: Origins and Uses of 8 Sign-Offs
1895, "finished, worn out, dead," from German kaputt, probably a misunderstanding of the phrase capot machen, a partial translation of French faire capot, literally "to make a bonnet," a phrase said in some etymological sources to mean "lose all the tricks in piquet" an obsolete card game. Popularized during World War I.
"Kaput" -- a slang word in common use which corresponds roughly to the English "done in," the French "fichu." Everything enemy was "kaput" in the early days of German victories. [F. Britten Austin, "According to Orders," New York, 1919]From French capot, literally "cover, bonnet," also the name of a type of greatcloak worn by sailors and soldiers, and faire capot also meant in French marine jargon "to overset in a squall when under sail." The card-playing sense attested in German only from 1690s, but capot in the transferred sense of "destroyed, ruined, lost" is attested from 1640s. [see William Jervis Jones, "A Lexicon of French Borrowings in the German Vocabulary (1575-1648)," Berlin, de Gruyter, 1976]. In Hoyle and other English gaming sources, faire capot is "to win all the tricks," and a different phrase, être capot, "to be a bonnet," is sometimes cited as the term for losing them. The sense reversal in German in the card-playing term might be explained because if someone wins all the tricks someone else has to lose them, and the same word capot, when it entered English from French in the mid-17c. meant "to score a cabot against; to win all the tricks from."
"There are others, says a third, that have played with my Lady Lurewell at picquet besides my lord; I have capotted her myself two or three times in an evening." [George Farquhar (1677-1707), "Sir Harry Wildair"]
Inoperative; ineffective; finished: I would be ''kaput'' without a folding machete/ Only sixteen of us. After that, kapoot
[1914+; fr echoic-symbolic German slang]