To go dutch (i.e. to share equally some expense or cost between two or more person). An uniquely Hong Kong slang prevalent in the 1960s and 1970s, and sometimes still heard today. Comes from the medical term to mean "in equal parts."
Let's go a/a on the 10-dollar meal. That's five from each of us.
Sex (chiefly British). Term popularized in the mid-1970s by British satirical magazine "Private Eye", which reported an incident on the vessel S.S. Uganda that involved illicit sex with schoolchildren on a holiday trip.