fan activities; supporting one's favorite idol or character
Slang term for the various activities fans do to support their favorite idol, actor, or character, such as buying merchandise, attending events, and promoting them online.
See also: 推し
I'm busy with fan activities for my favorite idol on weekends.
Sometimes I travel far for my oshi-katsu.
Compound of 推し (oshi, 'favorite idol or character') and 活 (katsu, 'activity'), a modern slang term from idol and fan culture.