In the American TV show, "Friends," the characters use "lobster" to refer to the person with whom you're supposed to be with forever. Ross tells Rachel that she is his lobster. Outside of this TV show (and references to the show), I have NEVER heard the word lobster used in that way. 99.9% of the time, when people (in the US, at least) talk about lobsters they are talking about the actual animal in the ocean. If you say "He's your lobster!" to someone who hasn't seen "Friends," they probably won't understand you.

I've never heard that. Probably, it comes from "Friends" the sitcom. Still, it's pretty slangy, I wouldn't use it in that way,