A passive-aggressive insult.
Usually used in situations and settings where swearing or blue language is not allowed, like corporate boardrooms, church, the family dinner table, etc.
Often allows speaker to call someone a jackass without the recipient of the insult even realizing what they are being called. Allows for a two-pronged verbal attack: 1) recipient is labled a jackass, 2) recipient suffers the embarrassment of being called a word that sounds like they should know the definition of, but don't. It this context, jackastic is a passive aggressive word.
Does the fact that you got cut from the cheerleading squad again explain why you are being so jackastic today?