Where does the term "sacred discard" come from?
I do not know the answer, but I asked in a experienced Mahjong group I'm in, kindly received a most valuable pointer and was subsequently able to extend it to this answer:


  Was it just a culturally interpretive term invented by English-speaking players?


Probably. An online search brough up http://i.imgur.com/crKOGnw.png (A Mah Jong Handbook: How to Play, Score, and Win, https://books.google.de/books?isbn=1462905005), which is from 1964 and may be a root source others copied from.

Note that Mahjong was introduced to Japan only in 1924, so it took some time for it to divert from the Chinese rules (and create Riichi), and further more for it to be adapted into English and into books. That this took about 40 years is not unlikely.

From the group's reaction it also appears that, depending where and with whom you play Mahjong, there is a fair chance you won't encounter the term 'sacred discard' at all, even though Wikipedia uses it occasionally (especially in introductory articles). Personally, I've also always seen, heard and used the original furiten instead (but I am not a regular player, so that amounts to not much).