A combination of the words 'insult' and 'insincere'. Something said in order to be offensive, while not necessarily being true or meant. Is total bull that is simply meant to insult someone.
Not to be confused with Insultsincere, which is when something is said that normally wouldn't have been in order to hurt someone. You mean what you're saying, you just normally wouldn't say it so harshly. Almost always a douche move.
Joey: Lisa, you're a fat bitch and I hate you.
Lisa: What? Please tell me you're just being insultcere!
Joey: No, I'm not. I abhor your existence. Lose some weight and stop being so damn obnoxious.