The toilets in the office building and the tube building are at one end of the corridor (at the other end, the stinky range is smaller), and it is often the first room on the floor, so sometimes it is euphemistically said "go to No. 1". There are other theories, but leave them at that.
There is also a number 1 in American English, but there is also a number 2, so what does the latter mean?
At least in American English, number 2 is "poop" and number 1 is "pee".
Although this is a euphemism, someone looked up the corresponding usage of late 1800s.
As for why number 2 is "small" and number 2 is "big", some people say that the two most important things that baby parents want their babies to learn are that they can pee and poop.
English number one can express the best, most important, or most successful person or thing in a group, such as.
Example sentence 1】Until his marriage, his job was number one in his life(His work was always a priority in his life until he got married.) [Longman Dictionary of Contemporary English].
Example sentence 2] shearson is number one in the market this year(Sheerson is number one on the market this year.) [Longman Dictionary of Contemporary English].
number two can express the second most important or second best person or thing, such as.
Example sentence 3: the party's leader is in f**our of the treaty but its number two opposes it.(The leader of the party was in favor of the treaty, but its number two was against) [Cambridge Dictionary of Senior Scholars].
Example sentence 4: After she beat Gomez she became world number two(After beating Gomez, she became the No. 2 in the world.) [Cambridge Dictionary of Senior Scholars].
Because babies learn to "pee" is the first thing they need to master, the number one, followed by "poop", is number 2.
Whether it is number 1 or number 2, the above semantics are equivalent to a verb, so an indefinite article (an uncountable noun in the example sentence [1,3, 4]) should be added, such as.
Example sentence 5】h**e you ever had to do a number two outside?(Have you ever done outside?)[Cambridge Dictionary of Senior Scholars].
Example sentence 6】you can go in the bushes if you need a number one(If you need to untie your little hands, you can go to the bushes.) [Cambridge Dictionary of Senior Scholars].
Example sentence 7】mum, i need a number two(Mom, I'm going to.) [Oxford Dictionary of Senior Scholars].
Also because the above semantic number one two is not a place, so there is no need to go (in Chinese, it is "上 去一").
As mentioned above, this is an expression of the infant context, and adults use it less (otherwise it is a bit of a giant baby).