underground: n subway (specifically underground railway): There’s an underground station two minutes from my house.
underlay: n carpet pad. As far as Americans are concerned, the “underlay” is the wood that lies underneath the carpet pad.
university: n college. As well as having the “University of St. Andrews” in the same way that Americans would have the “University of Oklahoma,” Brits use university as a general term to describe those sorts of institutions: I’m still at university at the moment. Brits do not use the word “college” in that context.
up the duff: n pregnant: Did you hear Judith’s up the duff again?
up the spout: n pregnant: I honestly didn’t mean to offend her, I thought she was up the spout!
uphill gardener: n homosexual. Perhaps best left at that.