Terms that were once used commonly can at a later date be deemed offensive. There are many examples, most of them with respect to a minority group, nationality, or ethnicity. PC can carry this to extremes, but "Jap" is today outside the PC realm and into common perception in the US as not being polite. "Limey" and "Yank" aren't at that level today, likely because the English and Americans don't feel like anyone whose opinion matters perceives them as being lower status, so they don't care much about it.
