Where did the "# years of pain" originate from?
It is actually a Japanese prank known as kancho.
From the Wikipedia article on Kancho.

Kanchō (カンチョー) is a Japanese prank performed by clasping the hands together in the shape of an imaginary gun and attempting to poke an unsuspecting victim's anus, often while exclaiming &quot;Kan-CHO!&quot;.
The word is a slang adoption of the Japanese word for enema (浣腸 kanchō).

According to Naruto Wiki:

This technique's full name is &quot;Konohagakure Hiden Secret Taijutsu Technique: One Thousand Years of Death&quot; (木ノ葉隠れ秘伝体術奥義・千年殺し, Konohagakure Hiden Taijutsu Ōgi: Sennen Goroshi, Viz: Konohagakure's Most Secret and Sacred Technique: One Thousand Years of Death, English TV: Hidden Leaf Village Secret Finger Jutsu: One Thousand Years of Death, UK DVD: Secret Ninja Technique of the Hidden Leaf School, Death for 1000 Years, English Games &quot;Sacred Taijutsu: 1000 Years of Death&quot;).

It is mentioned that Akira Shiotome (Miuna's brother) loves playing kancho:

He enjoys playing kancho with Hikari Sakishima and Shun Sayama and after Manaka awakens, the two enjoy playing with each other.

Also, in the scene where Akira played the prank in the 15th episode of Nagi no Asakura, he only said &quot;Kancho&quot;. The person who subbed it in English probably wrote it as &quot;# years of pain&quot; as a tribute to Naruto.