A person who is incapable of even the most basic math. (also: arithmetarded)
Chad: How much do we leave for a tip?
Kirk: Don't ask him, he's an arithmetard.
One who is a complete retard when it comes to even the simplest math. Especially applicable when that person should know math for their job.
I am such an arithmetard! I'm a tenured professor of theoretical physics, but can I calculate a simple tip? NO!