(Internet slang, especially anime and manga fandom) events or things from the past which one wants to whitewash/erase from history (due to it being embarrassing, poorly received, etc.); dark past

study of history

historic, historical / already-happened, past / former, old, traditional

(mahjong) a win worth four times of a 満貫 (mangan): 48,000 points if the winning player is on the East (dealer) side or 32,000 points otherwise

(mahjong) a winning hand using special rules; worth 48,000 points if the winning player is on the East (dealer) side or 32,000 points otherwise

(figuratively) driving force

(Japanese fiction, pornography) the one who fucks other characters

(military) reserve service

a tax that temples levied on property and residents / work done at a template; also, a priest who does such work

one who sets fire / (by extension) one responsible for something's popularity, etc.