"'What do you think you're doing? Those are for little girls you sick pervert'," a woman told him.
"'They're for my sister', I responded."
The woman, visibly angry, told the boy he should "have a female come and get them" so he didn't "do anything with them".
The teen said he tried to explain he was just grabbing the undies because his mum had asked him to but the woman was not having it.
"I get to the counter to pay for my items when she shows up again," he recalls.
"Don't let him have the underwear, he's a sick pervert," the woman shouted.
"They're for my sister," he told whoever could hear him.
Luckily for him, the shop assistant told the woman to back off.
"After that, I went home and give my sister the underwear.
"My sister is running around the house with just her underwear on.
"I think she likes them."
People on Reddit had a lot of sympathy for the boy and felt sorry he'd gone through that experience.
"Meanwhile, an actual pervert steals all the children's underwear behind the tirading mum," someone commented.
"Honestly, I feel like she might be the problem here if the first place her mind seems to have gone was to something sexual," another person said.