1)Used as an interjection, or an exclamation. Most commenly used to show good-natured displeasuer, or surprised amusement.
2)Is used by some as the 'final word' in an argument. Occaisionally used as a substitute for a word one can't recall, although this is generally frowned upon.
3)It is, however, completly acceptable to use shnaw in place of profanity when in situations that cussing is totaly unaceptable.
Originated due to the poor verbal skills of a budding thespian, who couldn't say "Mrs Shaw" after recently talking to "Mr Snell" while in a play; every time it would come out as "Mrs Shnaw."
The word then came a 'catch-all' word to the students in the class before spreading to the school.
Rob: I can't come over tonight. It's my mom's birthday.
Jess: -In conclusion, although the Guardsmen at Kent State may have been provoked, they were not in such danger as to justify lethal force.
Aaron: Yeah, well, shnaw!
Tom: (Dropping the communion wine in church) Oh. Shnaw.