To urinate. Also see number 2

"This will be quick, I just gotta' take a number 1."

1) This term is used to describe something that is the best in it's respective class.

2) It can also be used when someone is talking about what is most important to themselves (normally themselves).

1) Girl - Why do you keep touching and looking at my ass, it's not that good

Boy - It is baby, I love it that's all cos it's number 1

2) I love her, she always comes first because she is my number 1 priority.

to always be looking out and paying extra close attention to the wellbeing of ones self and not care about those around themselves

i see that tim blyth's (no vowel!) looking out for number 1 again i see