It is when you put an expense in terms that a girl can understand. If it costs $300.00 to get your brakes fixed, that is 4 pairs of shoes and three tubes of lipstick in girl math.
Kenzie: A: OMG! That speeding ticket is going to cost me $200.00!
Amber: That is like 2 pairs of shoes, 1 bottle of nail polish and 2 packages of skittles and a diet coke in girl math.
When a pair of cute shoes retail at $89 and you find them on sale for $29; you did not save $60, yet this means you have $60 yet to spend!
Cassie: OMG, these are only $29??
Melissa: Nice, what's the retail?
Melissa: You have $60 left to spend!
Melissa & Cassie: GIRL MATH!!
Arithmetic which is only relevant to females.
He exasperatedly reminded her: "Don't ask me the ages of our children, or how many years we've been married -- that's girlmath."