When I was being adopted from an orphanage my mom claims that on several occasions, when calling for my little brother I called him by a nickname. I don't know how to spell the word or what it means I just know how to say it. I would call him something along the lines of Sideosha. That's the only way I can think to spell it as that is how it's pronounced. Any help would be greatly appreciated as it's been driving me nuts for years.
Another (admittedly less likely) possibility would be that the name is a diminutive form of Сидор. While I'm not aware of a standardized diminutive form for that name, kids make up the darndest things.