Wikipedia says it used to stand for mistress. But you don't read it as mistress, do you?
What is the pronunciation like? I think it would depend on the expansion.
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The title Mrs. is pronounced /ˈmɪsɪz/ (MISS-iz). It is not ever spelled out to anything in formal contexts.
In informal contexts, it is often spelled out as “missus”. There are 258 incidences of “missus” in the Corpus of Contemporary American English, nearly all in fiction, except for a handful in quotations in magazine and newspaper articles.