WC: Water Closet n. 盥洗室，厕所

厕所
Some of the ways we refer to the place where you relieve yourself in English include:
Toilet - Any place that has an actual toilet. 马桶
Restroom - Often in a restaurant or public place. 公共场所的厕所
Bathroom - Inside someone's house. 家里的厕所
Men's room or ladies room 男士，女士卫生间

大小便
Some of the idioms we use in English to express that we need to relieve ourselves are:
Hit the head
Hit / Use the John
Porcelain throne
小号 drain the main vein 仅限男生用，略微粗鲁
大号 Dropping a deuce
大号 Take a dump

放屁
There is also several ways of saying fart in English, some are more acceptable than others.
相对较礼貌
To pass gas
Toot
Breakwind
较粗鲁
Rip one
Cut the cheese
粗鲁
Shart -- When you think you have to fart but you actually have a little poop come out.

问厕所
口语化，适用任何场景，不会太正式也不会太粗鲁
D- "Excuse me miss where can I find the restroom?"
C- "It's the third door on your left sir"
比较随意，和亲近的朋友使用
D- "Hey John, where is your toilet?"
C- "Just down the hall."
非常正式
D- "Is there a place where I can relieve myself"
C- "Yes, the second floor has a lavatory"