Q: Lawyers – and they aren’t the only ones – prefer words with more syllables than necessary, perhaps because they think it makes them sound more professional. I don’t know when it started, but they now refer to co-conspirators, though probably not to their co-colleagues, co-partners, and co-companions.
A: I’ve noticed this hyper-syllabic talk myself, though I’m not particularly bothered by the term “co-conspirator.”
Lawyers and other professionals do indeed like Latin words and affixes (prefixes, suffixes, and other attachments). In fact, I touched on the subject in a blog entry a while back.
However, people have been referring to co-conspirators for a lot longer than you seem to believe – for about a century and a half!
The first published reference to the usage in the Oxford English Dictionary is from an 1863 book about Spanish America: “He has sought to become … in the palace of the French emperor a co-conspirator with him.”
As for other “co” words, the OED has dozens of citations since the early 1600s with the prefix attached to nouns, verbs, adjectives, and adverbs. In general, I wouldn’t describe these prefixes as unnecessary.
For example, in The Antipodes, a 1638 comedy by Richard Brome, one of the characters in a play within the play is asked “to speake to your co-actors in the Scene.”