Fingers in his ass is a meme mainly quoted/said by Danganronpa fans on a site called tumblr or Danganronpa fans in general.
The sentence is usually associated with the pairing sansmaeda. There is also a thing called fingers in his all sunday. On Sundays people gather to appreciate the song fingers in ass and sansmaeda. On December 16th 2018, the day before tumblr's adult content ban fans of sansmaeda and fingers in his ass sunday celebrated what was called the "final" fingers in his ass sunday.

Origins:
On july 17th, 2017 user named skeletortilla uploaded a video on tumblr that had Nagito Komaeda from Danganronpa 2 and Sans from Undertale dancing together to a song called "ASSHOLE (Fingerfückung)" made by BotanicSage on youtube.
Kanye west he likes fingers in his ass
It's fingers in his ass sunday!
by Pupseus December 17, 2018
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Fingers in his ass originally was a joke from both the undertale and danganronpa fandoms and features the characters Sans Undertale and Nagito Komoeada.
Fingers in his ass, fingers in his ass, Kanye west he likes, fingers in his ass.”
by BrotherBasil January 7, 2022
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Basically, a joke that originates from a shitty video in which sans and nagito komaeda dance with the hallway that has sans in it in the background, with the song "fingers in his ass" playing in the background, which was discovered on a Sunday, afterwards creating Fingers in his ass Sunday.
"damn, it's fingers in his ass Sunday! Time to watch that video again!"
by Jeef The Dumbass May 16, 2022
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