There is a Jewish surname Saide; this may be another spelling of the German Seide, meaning "silk". Seide itself is a Jewish name, as is Zajde, which looks like a Polish spelling. As for the pronunciation, German Seide would be pronounce ZYE-deh, which is one reason to be cautious about the above explanation. Why isn't it spelt with a Z instead of an S? I suppose it depends where this name originated, modified by where it ended up.