Telaine Smith, 31, from Queensland, Australia, was parked in a hardware store car park when a woman aggressively banged on her window and made the comment.
Smith was left speechless by the woman's rudeness, but her six-year-old son Dallas, who was in the backseat with her four-month-old son Pilot, told the woman: "If you don’t want to see it, you don’t have to look."
Pilot was born tongue-tied and is unable to latch on to the breast, so Smith has to express milk for about 30 minutes every two to three hours.
Smith told local news site Fraser Coast Chronicle that the worst thing about the incident was that getting tense made it harder for her to express milk.
She said people often seemed to think breastfeeding and expressing milk were "confrontational" acts.
"People see it as sexual," she said. "They have to realise it's not."
Smith told Daily Mail Australia that although until now she has never experienced "such a level of aggression", she had previously felt judged while she expressed or bottle fed.
"People ask questions when I do it in public," she said. "In the baby rooms when I’m feeding with a bottle … [some mothers] get their noses out of joint assuming I’m feeding formula.
"You feel like you need to justify yourself, but I shouldn’t have to."
Smith added that she'd like to encourage other mums to feel confident about however they feed their baby.
"We should support each other regardless," she said. "Being a mother is hard enough."
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