Wasei kango (和製漢語), orthographically borrowed from Japanese 便(べん)所(じょ) (benjo).^[1]

便所

1. (chiefly Northeastern Mandarin, Taiwanese Hokkien, Taiwanese Hakka) bathroom; restroom; lavatory; toilet

   便所佇佗位？ [Hokkien, trad.]
   便所伫佗位？ [Hokkien, simp.]

   Piān-só͘ tī tah-ūi? [Pe̍h-ōe-jī]

   where's the toilet?

Appears to be a coinage in Japanese^[1] from Middle Chinese-derived roots, as a compound of 便 (ben, “convenience, ease”, also meaning “bodily waste; bowel movement”) +‎ 所 (sho, “place”). The sho changes to jo as an instance of rendaku (連濁). Compare English expressions seat of ease, house of ease, also pooper, shitter.

First cited in works from the late 1400s.^[1]

• (Tokyo) べんじょ [bèńjóꜜ] (Odaka – [3])^[2]
• IPA^(key): [bẽ̞ɲ̟d͡ʑo̞]

便(べん)所(じょ) • (benjo) 

1. [from late 1400s] (dated) bathroom, restroom, lavatory, toilet
2. (gambling slang) a pachinko machine one can use leftover balls on.

While up until the 1970s, this term or the honorific お便所(べんじょ) (o-benjo) was the preferred polite term to refer to a bathroom, it has now been largely replaced with お手洗(てあら)い (o-tearai, “washroom”)—or even トイレ (toire, “toilet”)—and sounds dated.

• トイレ (toire), おトイレ (o-toire)
• 手洗(てあら)い (tearai), お手洗(てあら)い (otearai, “washroom”)
• 化粧室 (keshōshitsu, “lavatory”, literally “make-up room, powder room”) – used in public spaces, like shops or airplane, more as the written sign than in speech.

• お便所(べんじょ) (o-benjo)

便所 • (byeonso) (hangeul 변소)

1. hanja form^? of 변소 (“bathroom, restroom, lavatory, toilet”)