Usually, when English borrows a word from some other modern language, we keep the spelling used in the source language and hesitate about the pronunciation. Thus we all agree on the spelling restaurant (from French), but disagree on how to pronounce the last syllable. We may take a cavalier line with diacritics, as when Swedish smörgåsbord becomes just smorgasbord. And with languages not written in the Roman alphabet we use a romanization, thus perestroika or tsunami. But generally speaking the spelling is not controversial, though the pronunciation may be.
There’s a word ʃtʊm that has become quite well established in the UK (or perhaps particularly in London and environs; I don’t think Americans ever use it). There is no question about its pronunciation. But we can’t agree on how to spell it. This is the other way round from what is usual.
The word means ‘silent’, and is used almost exclusively in the phrase keep ʃtʊm or its variant stay ʃtʊm ‘keep quiet (about something)’. (The OED also offers us a verb, to ʃtʊm up, but I don’t think I’ve ever heard that.)
We agree on how to say it: but how do we spell it? There are quite a few candidates.
Rather than attempt to interpret the mysteries of Google’s hit statistics, I searched the Guardian newspaper website for various possible spellings of the word.
184 hits for schtum,
78 for shtoom,
41 for shtum,
21 for stumm (but some of these are for a proper name),
15 for stum (ditto),
5 for shtumm,
4 for schtumm
and 3 for schtoom.
The OED’s first citation is dated 1958. The origin of the word is Yiddish, the equivalent of German stumm ʃtʊm ‘dumb, speechless, mute, silent’. (That’s ‘dumb’ in the older sense, ‘unable to speak’, not the modern AmE sense ‘stupid’.) But we see that the spelling used in German, stumm, comes only in fourth place in the Guardian statistics. In Yiddish it’s spelt שטום, which transliterates as shtum (please correct me if I’m wrong), and this transliteration-spelling is in third place.
In German the word-initial spelling st- corresponds to ʃt. The English spellings with scht-, which look German to us, aren’t German at all.