Provenance — where it came from
推す (osu) is an ordinary verb: to push, to put forward, to recommend. Nothing about it suggests devotion.
Idol fans changed that. By the 2000s, a fan of a large idol group needed a word for the one member they backed above all others — the one whose CDs they bought twice, whose handshake line they stood in. That member was the one you “push”: your 推しメン (oshi-men, “push member”), soon just your 推し (oshi). In the 2010s the word escaped idol culture entirely. An anime character can be your oshi. So can an actor, a sumo wrestler, a voice actor, a shrine.
By the early 2020s it was mainstream Japanese. The activity of supporting one’s oshi — buying, attending, collecting, evangelizing — got its own word, 推し活 (oshi-katsu), and in 2021 a novel titled 『推し、燃ゆ』 (Idol, Burning) won the Akutagawa Prize, Japan’s best-known literary award. The word has begun to travel into English through fan culture, as otaku did a generation earlier. A plain verb for pushing, now carrying a whole economy of love.
Field notes — how it’s used
推しが尊い
oshi ga tōtoi
“My oshi is precious.” Religious vocabulary, used without irony.