As I was falling asleep yester-day morning, I was thinking with annoyance about the term
conspire comes from the Latin
conspirare, which literally means breathe together, and breaks into
con- from the Latin preposition
com meaning with, and
spirare, meaning breathe. So far, so good.
Things run off the rails with that
co-conspirator. The prefix
co- is really just a reduced form of
com-. Part of the reason that the reduced form is used here is that the original morphology of
com- was simply forgot, and whoever coined the term reached for analogy with some term formed by a chain of analogies ultimately leading back to a word that used the reduced form as per rules of Latin morphology (such as
co-author). Had that person remembered the morphology — had he or she recognized
com- — he or she might have seen the deeper problem.
com-, and one gets … uhm,
conconspirator; crudely parsed, that's with-with-breather. That result should raise a warning flag. One should ask whether there is any difference between a conspirator and a co-conspirator. It isn't possible to be in a conspiracy of one (though the claim might be made jocularly).
I think that the term
co-conspirator first came to general use during the Watergate Era. Certainly, I don't find the terms
co-conspirator in the American Heritage Dictionary of 1975. I'd guess that the term
co-conspirator was probably coined by a lawyer, and that it lived for some time in the environment of the court-house, before escaping into the wild exactly as a result of President Richard Milhous Nixon's being called an
unindicted co-conspirator in court documents.
I'm reluctant to condemn people who, raised in the years since, use
co-conspirator without irony. Even if they recognize the absurdity, it is difficult for people to distinguish those absurdities that one must accept from those from which we might more easily be freed. And I suspect that, in many cases, the folk who use this
co- are really trying to capture the sense of fellow; though that sense would be better captured with, well,
fellow, at least the
co- isn't then wholly redundant. But, really, we ought to make an effort to drive this thing from our language.
 In Latin, normally, the reduced form
co- is used when followed immediately by a vowel, by
h, or by
gn. The basic form
com- is used when immediately followed by
m, and by
p, but it is assimilated into
l and into
r, and it becomes
con- in front of the remaining consonants. Things get less consistent when the construction was not actually made in Latin. Meanwhile, in Latin itself the earlier preposition
com evolved into