Why are “sugar” and “sure” pronounced with an SH?

Question

As far as I know, those are the only two. They should be pronounced Soogher and Soor, shouldn't they? I looked them up on Dictionary.com, and their etymologies reveal no trace of an SH, except where the listing for sugar had:

Middle English sugre, sucre (noun) < Middle French sucre < Medieval Latin succārum < Italian zucchero < Arabic sukkar; obscurely akin to Persian shakar, Greek sákcharon

I see an obscure kinship to shakar, but the word morphed so many times since then that the SH disappeared pretty much completely. And sure is even worse, with no sign of SH:

Middle English sur ( e ) < Middle French sur, Old French seur < Latin sēcūrus

Why are these the only two like this?

Answer 1

This interesting page explains that sugar used to be pronounced originally with a common su sound, but (emphases mine):

(...) sometime in the Middle English period the initial letters su shifted to the pronunciation they now have.

If you relax the mouth and tongue somewhat when you are saying the older form, your pronunciation shifts to the modern one, as you’ll realise if you try out the two sounds in turn; the modern version is actually rather easier for slack-jawed English speakers to say. (...) The same change happened with other words, such as sure, and also to words in which the sound occurred in the middle, such as pressure and nation. By the time this shift in pronunciation was taking place, the spelling of the words had already become fixed (...)

Answer 2

The sound of French /u/ (a close front rounded vowel) is denoted [y]. Sugar is from French sucre [sykʀ(ə)], and sure is from French sur (e) [syːʀ].

Middle English kept that vowel originally—as [sykrə] and [syːr(ə)], respectively—but over time it was transformed into [ju] (like modern you). What was previously [sy-] thus became [sju-] (RP sue) and eventually [ʃʊ-] (as in modern sugar).

This happened in other contexts: -tion is pronounced [-sjɔ̃] in French, with a nasal vowel. This became [-sjɔn] (syon) or [-sjən] (syun), and finally [-ʃən] (shun).

Answer 3

I've noticed many Scottish and Irish Gaelic words to be spelled with an s, followed by a vowel, and pronounced like sh. Think about the way Sean Connery speaks (not to mention how the Se in his name is pronounced).

Read the section on nomenclature here and click on some of the links about Anglic and Scots (Yes, I realize I'm quoting Wikipedia, but there is a source attached to this quote). Note how the s is pronounced sh:

Prior to the 15th century, the Anglic speech of the Lowlands was known as Inglis ("English"), with Gaelic being called Scottis ("Scottish"). From the late 15th century, however, it became increasingly common to refer to Scottish Gaelic as Erse ("Irish") and the Lowland vernacular as Scottis.

There is also some good information about Scottish Gaelic orthography and phonology that you might find useful if you can read IPA.

Answer 4

The first part of 'sugar' used to be pronounced more like ''sue''. The vowel in that syllable is a high, back vowel and sometimes consonants change to be more like the vowels that are near them. It's called assimilation.