Sourced from an old (currently-defunct) Pixiv Encyclopedia article.
1. Abbreviation of "Umapyoi Densetsu", one of the songs for the Umamusume franchise.
2. To dance or sing along with the Umapyoi Densetsu song.
3. [analogy from 1.] To fully finish a given Umamusume's training scenario (winning the URA finals with first place) in the Umamusume Pretty Derby game.
4. [analogy from 3.][euphemism] To "go all the way" with a given Umamusume. To achieve last (fourth) base.
Slang coined by the Umamusume fanbase, deriving from the title and song lyrics of the Umapyoi Densetsu song. Generally used in the sense of: 1. To finish a given umamusume horse girl's training scenario in the Umamusume game; 2. To get along very well or "buddy buddy" with a given umamusume (or Kiryuuin Aoi); or 3. To "go all the way" or do all sorts of implied hanky panky things with a given umamusume (or again, Kiryuuin Aoi).
Effectively used as a euphemism in most fan comics, especially in 'comedic' scenarios, where it usually involves an umamusume going yandere on the Trainer and alleging the latter umapyoi-ing the other girls. Occasionally it is Happy Meek accusing Kiryuuin Aoi (of going out with the the Trainer), as a reference to one of the in-game events.
A common variation used by fans (but not in official media) is "onsen umapyoi", occasionally abbreviated to "onsenpyoi" (温泉ぴょい). This can refer to either: 1. to finish a given umamusume horse girl's training scenario with the semi-secret hot spring event unlocked, which requires the player to randomly acquire the Hot Spring Tickets in that scenario and win the URA final, or 2. from the above, to "go all the way" at the hot spring ryokan with a given umamusume (or, yet again, Kiryuuin Aoi).
Since the release of the Aoharu training mode, Kashimoto Riko has joined Kiryuuin Aoi in the list of possible umapyoi-ees (or, alternatively, umapyoi-ers).
The song also features other 'nonsensical' portmanteau words such as umadacchi, sukidacchi and umapoi (without the 'y'), all of which are also co-opted by the fanbase for pretty much the same meanings as umapyoi above. As such, these examples can also be tagged umapyoi.
Usage of all these four words has also spread to other fanbases.
This tag should generally not be used when the words occur as part of the Umapyoi Densetsu song, i.e. the characters are singing the song normally in their original context.
How the words are spelled (for ID-ing)
- うまぴょい umapyoi, "horseyhop!"
- うまだっち umadacchi, "horseypals!" or "horsey-stand-up!"
- すきだっち sukidacchi, "likeypals!"
- うまぽい umapoi, "horseytoss!"
- 우마뾰이 "umapyoi" in Korean
Original song lyrics, first verse
うまうまうみゃうみゃ 3 2 1 Fight！！
Uuuuuuu (Umapyoi Umapyoi!)
Uuuuu (Sukidacchi!) Uuuuu (Umapoi!)
Uma uma umya umya 3 2 1 Fight!!