Please Excuse My Dear Aunt Sally

In school it means PEMDAS which is the order of operations:

Parentheses
Exponent
Multiplication

Division
Addition
Subtraction

When solving an algebraic equation, you'd use PEMDAS to help you figure out which part of the equation to calculate first, second, third, etc.

Could also refer to your aunt named Sally who lacks self control, so please, excuse her.

Teacher: "Alright, kids! Today we're going to learn PEMDAS. That means please excuse my dear aunt sally."

Kid: "What the fuck, man? You guys put letters into math and now we're writing sentences? I was in English last block. Fuck school."

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