Inspired by comments on Proper use of the word “lousy”?:
The word lousy is traditionally pronounced with a /z/ sound, as though it were louzy.* Contrastingly, the word mousy is always pronounced with an /s/ sound. The difference seems to be pretty consistent and well-established: these are the only pronunciations listed in the OED and Merriam-Webster for each.
But their root words, louse and mouse, are pronounced identically, both with /s/. Indeed, these words are parallel in almost every other way: they form analogous plurals — lice, mice — and have very similar origins.
So how and why did the pronunciations of lousy and mousy diverge? And are there any other analogous words that also form analogous adjectives? (House doesn’t form *housy; and blousy exists, but blouse is not analogous to the other words.)
*: It seems that recently, since the literal meaning “infested with lice” has become rare, it may sometimes get pronounced with /s/ in this sense, as a spelling pronunciation.
Best Answer
The answer is historical. In Old English, voiced and voiceless fricatives were phonologically equivalent. (The current English graphemes <v> and <z> did not exist in Old English.) Between voiced segments, voiceless fricatives became voiced--and this is called sibilant softening. Then, in Middle English, these fricatives became phonemic. That's why you see these discrepancies. Words with <s> between voiced segments after the Middle Ages don't fit in this pattern. That's why lousy (from the 14th cy) got /z/ (softened /s/), whereas mousy (from 1853) got /s/.