Sheltered an upbringing to be aware of

In trying to explain the origin of 'Cock a snook' it would be helpful to know what a snook is. Unfortunately we don't really. There is a species of fish called snook, but it isn't that, unless there's a form of derisive gesture that I've had too sheltered an upbringing to be aware of. A snook is also a promontory of jutting out land. That could have something to do with the gesture as it does involve sticking fingers out. Apart from this single phrase, snook isn't a word you would expect to hear very often. It is sometimes reported to be derived from snout, as in thumbing one's nose. That's possible but, although snout and snook are somewhat similar, why didn't they just 'cock a snout'. That term doesn't appear to be recorded.

Please explain to me the highlighted parts.

Source : http://www.phrases.org.uk/meanings/cock-a-snook.html