why are you replying to threads that haven’t been posted on in like 4 months

 

i have brain damage dont talk to me or i will find you

 

is that a threat I will report you OMG

 

no its not am sorry for stealing please forgive me

 

Insults require context. That's why people can ruthlessly grill their friends, but what a dude says to you on the street doesn't really count for anything. Insults have to be harsh truths to be effective, otherwise, they're basically meaningless.
For general ones, you can always go with the classic retort that they're projecting their insecurities-- "you have an IQ of 5" -- -"takes one to know one" or "it's okay to be insecure about your intelligence, but leave other people out of it". These kinds of subtle insults are pretty safe and witty, and ensure that you're only ever dishing out what you get, rather than attacking them needlessly. This being said, they're not very original.

 

Ill grill your ukulele strings

 

Were you born on the side of the freeway? Cause that’s where most accidents happen.

 

you have nice clothes