Err - the shortened or "quicker" way of saying Error in the context of a payout/insult.
Err has two intentions.
1. To insult another on a mistake or stupid comment they said.
2. To make another person out to be mentally retarded.
To fully understand the use of Err, Herp-a-derp
is helpful as it is the same type of insult. Err is a very flexible payout and can be paired with a number of other insults. The most common is the use of "Kid" at the end to form Err kid! Kid has no second meaning other than to "be-little" its intended subject.
Finally the use of Err, also has a physical aspect. The use of a hand gesture is "the icing on the cake" when using this insult, especially if it is also paired with "Kid" as above. To describe this hand gesture, it is kind of like a Karate chop, stopped halfway through leaving the hand pointing at the person you intend to insult. An extra note is to use agression, you cannot be too big with this payout, thats why its so wonderful :)