A Scots noun meaning "a little something", "a bit of scran" to have as a snack, especially with a cup of tea between meals. Believed to be a contraction of "it is nice to", as in: "It's nice to have a biscuit with your tea." It's colloquial, so spelling is unknown. Sometimes (erroneously) pronounced with a "sh" sound, as in "schneister".
"Do you fancy a wee sneister with that, son?"