I'm talking about the Latin cum, which I've seen used conjunctively, as in A-cum-B. What does it mean, and how do you use it?
About that matter, Etymonline says of the use of the Latin cum preposition:
The sexual cum seems to have no connection with Latin cum, the preposition meaning “with, together with”, which is occasionally used in English in local names of combined parishes or benifices (e.g. Chorlton-cum-Hardy), in popular Latin phrases (e.g. cum laude), or as a combining word to indicate a dual nature or function (e.g. slumber party-cum-bloodbath).
It's to be noted that both have the same pronunciation (/kʌm/ in British English, /kəm/ in American English). Both Etymonline, the New Oxford American Dictionary and Cambridge Advanced Learner’s Dictionary have it written “X-cum-Y”, with hyphens. Funnily, the later two use related examples, NOAD going for “study-cum-bedroom” and Cambridge for “bedroom-cum-study”.
However, modern usage doesn't always follow that prescription, at least in American English. The Corpus of Contemporary American English data for 2010 shows 11 occurrences using hyphens, and 3 occurrences without hyphens (such as “lawyer cum fitness enthusiast”, right there).