Why "sh" pronunciation in "sugar"?

[Douglas G. Wilson]

>   Today a student asked me why "sugar" and "sure" are pronounced with
> "sh" (vs. "super" with "s"). Would anyone know?

I shurely wouldn't. But here are my puerile observations anyway.

The palatalization /s(j)u/ > /S(j)u/ is quite common in unstressed
syllables: "issue", "tissue", "fissure", "censure", "pressure",
"Assurbanipal"/"Ashurbanipal" (?), etc.

Similar change in stressed syllables is more unconventional; it occurs in
the 'special cases' "sugar", "sure" ... and in derivatives: "sugary",
surely", but also "assurance" (but not "assume" or "assurgency").

Offhand I can think of one word -- "sumac" -- where the stressed syllable
can go either way ("soo-mack" or "shoo-mack").

-- Doug Wilson