what does "put starch in pinafore"mean? does it=oatmeal? i don't get this sentence
"My stomach hurts." It was hardly a lie anymore. Her stomach was feeling worse by the minute—sort of a dull pain with a little sharp point in the middle.
"Well, of course it hurts," Grandma chimed in. "It's empty. You run and dress before your nice oatmeal gets all cold. It'll put starch in your pinafore! Turn you into a new woman. Guaranteed."

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Answer

A pinafore is like an apron. Starch is a chemical used on clothing to make it stiff without wrinkles. "It'll put starch in your pinafore!" in this context means something like, "It will make you tougher/stronger!"