Granted, this isn't as exciting
as any of the above writeups, but the usage of "cum" you are most likely to encounter in legitimate speech is actually an extension of the Latin
usage into English
"Cum" is used now to denote a dual function or usage for some noun, as in:
Jerry walked into his garage-cum-studio.
We see our protagonist Jerry walking into a place which is used as both a garage and a studio (for his post-punk noise band, The New Banana, if you must know). You get the idea.