I learned some of the unusual (not used daily) terms in English from the strangest places.
(1) booboo (minor scratch or scrapes) - believe it or not, during my internship at the County Coroner's Office
(2) It's not over till the fat lady sing - only place I've ever heard or used it -- playing spades online
(3) Navel (belly button) and tympanic membrane (ear drum) - anatomy class, okay it was a strange place to learn the medical term, the strange thing was I didn't learn the layman terms till way later. And people thought I was being pretentious. :P
(4) yaddi yaddi yadda - Seinfeld, someone once asked in awe, "Did you know that's a Jewish saying?" I guess Seinfeld is Jewish.
Other miscellaneous terms - various sitcoms. Once in a while someone would ask in horror, "Did you know it has __________ connotation?" Heck would I know? I picked it up from a sitcom!
That said, I think the best way to learn common sayings are to ask the people who used it. They either hate you or they don't. Some people just get used to me asking (and explaining to me), others will avoid using what I'd ask altogether (maybe they were being polite?). But I like learning stuff that you don't find in a textbook, so I am going to continue to ask, ask, ask. :D