There is an English expression do for, which means to kill, to execute, to ruin, to defeat etc. and this expression seems to always be used in passive voice: e.g.) We are done for.
I understand this is like an idiom, but why is the preposition for used?
Most prepositions have so many meanings to them, and I would like to know what for in this case means.
edit:
https://dictionary.cambridge.org/us/dictionary/english/for
https://en.oxforddictionaries.com/definition/for
In the definitions of the above links, which for do you all think the for in done for is the closest to?
edit 2:
I don't think the for is dangling, but I just want to know why for is used. Like, for normally means purpose, cause, or to be given, etc.
But the for in question means quite opposite.
Best Answer
"For" at this point doesn't seem to have any meaning here, although it does add a little emphasis.
Specifically, it looks to me like "done for" could be actually a variation of the even older, now-obsolete verb "fordo".
The earliest example in the OED for "done for" is from a1500:
"[Lansd. fordone]" means that in another manuscript they used "fordone" instead of "don[e] for". (Unfortunately I haven't been able to access either version.)
If you look at this free definition for "fordo" (for Middle English, but still applicable), you'll see it is essentially a synonym for "done for". "Fordo" is ancient (first attested in English in the year c900) and apparently derives from "Proto-Germanic *fardōną, *fradōną (“to ruin, destroy”)".
According to the OED, the "for" prefix in "fordo" means:
It's worth mentioning that the now-obsolete word "fordeem" is a synonym because it means "to ruin or destroy", although it also means "to condemn". There are other obsolete words that also have this "for" meaning destruction prefix but I can't think of any current ones. Of course there are plenty of current verbs with other senses of the same "for" prefix: forgive, forget, forbid, etc.