Having been somewhat of a no-life AOL/Internet In General geek (yes, admittedly) I ran across this Straight Dope article - "What is a “snert”?
Describing the term as an insult is vaguely correct, usually though people are dubbed “snerts” when they are caught lying about their life OFF the internet.
For example, bragging about your significant other’s 6 digit income and your beautiful 3 story house, then making the tragic mistake of emailing or posting pictures of your not-so-lovely 1 bedroom apartment with a view of the back alley behind Winn Dixie and captioning them “Our Christmas Portrait with Princess and Duke” (being your two three legged “champion bloodline” mutts)
Another example are people (generally either teens or posing as teens) posting same elaborate stories of their overly wonderful lives and then sending out pictures of OTHER people’s children as their own.
9 times out of 10 though a “snert” becomes a huge insult when the accused has been a real snot head - but not simply BECAUSE of their having acted like one!
In times like those, the accused “snert” will rarely admit to lying and continues to fling insults -as well as false stories and stolen/fabricated/completely disproving pictures if they don’t refuse flat out to send more to back up their claims.
Or - in a completely opposite situation, they’re caught red handed and continue to play dumb.
