fan of an entire group; group stan
Idol-fandom slang: supporting every member of a group equally, rather than having a single favorite. Often contrasted with 単推し (tan'oshi, 'single-member fan').
I'm not a fan of a specific member; I'm a fan of the whole group.
Since I'm a group stan, I'm happy no matter who is the center.
From 箱 (hako, 'box', here metaphorically 'the whole group') + 推し (oshi, 'favorite, support'), a noun form of 推す (osu, 'to recommend, support'). The term originated in idol-fandom slang.