okaerinko

おかえりんこ
This difficult-to-translate pun comes from stories of practical jokes long told on 2channel.

The archetype, as retold in Danbooru post #116261: ======
Upon her return home, I said to my little sister - an 8th-grader - "Okaeri-nko."
("Okaeri" means "welcome back".)

She started to respond "Tadaima-nko", before suddenly turning bright red and hitting me, saying "Geez! You pervert!" I got a huge kick out of her innocent reaction.
("Tadaima" means "I'm home", and "manko" means "vagina".)

Later, when my older sister came home from work, I said "Okaeri-nko" to her, too. Without the slightest change in expression, she responded "Tadaima-nko. What's for dinner tonight?" Impressed, I couldn't think of a thing to say.
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This story was already in existence as of 2003. As it has been spread around, various small variations have shown up.
It's particularly popular among boys with older sisters, and there are people who have actually tried it.
"Okaeri-nko" is probably a variation on a the slang word "tomodachi-nko", which is found in Kobayashi Yoshinori 's manga "Obocchama-kun."
("Tomodachi" means "friend" and "chinko" means "penis.")