As "lousy" is a pejorative for someone or something being infested with lice, why is it pronounced with a "z" instead of an "s" sound?
OTOH, when a person is called a "louse," the "s" sound is used.
Doubtless most people who use the word (lousy) don't really know what they're saying, thus the mispronunciation, but how did it get started in the first place?