Telling someone that they are awkward is an insult. Calling someone anus is an insult.
Insults that start with the letter d:daffydensederangeddimwitteddisgustingdishonestdopeydowdydrunkdulldysfunctional
jerk
Quitter!
insults starting with the letter L
insane
Vent-head.
Moronic is an insult. It begins with the letter m.
rectum sucker
Asinine is an insult. It begins with the letter a.
insular, insults, insured...
There are many websites and blogs that have lists of insults including Buzz Feed, which features Shakespearean insults to Wikipedia, which has a list of ethnic insults.
9 letter words that begin with the letter i:identicalignoramusillegallyillegibleillogicalimaginaryimitationimmediateimmersionimmigrantimprudentinclusioninclusiveinfluenceintrusive