Fundab is an antonym for "Fuck You In Da Butt". This word is used when you want to tell someone to "fuck themselves in the ass" but can be used more quickly and in mixed company. This word is pronounced Fun Dab.
Boss: "I need those TPS reports on my desk by 5 o'clock."
= You told your boss to go fuck himself in the ass and he/she is none the wiser.