From the Italian, meaning 'low voice'. To speak quietly out of the corner of your mouth so that the subject of your speech can't hear your insult or know that you are speaking.
"Oh, what a beautifulcreature your daughter is", said Mrs. Mulroy saccarinely to Mrs. Katz at the Bat-mitzvah.
"Too bad the zoo can't identify what kind of creature she is", Mrs. Jackson said sotto voce to Mrs. Mulroy as they plastered wide smiles on their faces.
Like sotto voce (commenting softly so that the third-person subject of the comment cannot hear the comment),
but spoken loudly with the intention that the third-person subject will hear that he is being spoken about.
A reversal of sotto voce: sotto-sotto intended to mean a loud voice.
Person 1 speaking to person 2 with person 3 in ear-shot (the subject of the remark):
In sotto-sotto voce: "Someone doesn't know their place!"