Its used as a verb. It is what you actually do when you get to work. Many jobs have fancy names like Assistant executive vice-president to the blind foundation of excellence, but people dont know what that means, so you ask them, "whats your nine to five?" and they tell you if they sit at a desk, or do they write papers? are they walking around supervising people? what does the person actually do all day, the actual 9am to 5pm shift, what are they doing specifically?
When you are out partying and you bring home a girl that is a nine, but you wake up the next morning and she is a five laying in the bed next to you. You just experienced a nine to five.
Joe thought he brought home a fine bitty from the club last night, but when he woke up, there was a five in his bed. Joe got nine to fived.
Also used in the Ramonessong "It's Not My Place" on the Pleasant Dreams album
My mom and dad are always fighting
And it's getting very unexciting
To get a good job You need a proper schooling
Now who the hell
Do ya think you're fooling
But it's not my place oh no
No it's not my place no no
No it's not my not my not my place
In the 9 to 5 world
And it's not my place
In the 9 to 5 world
And it's not my place
with 9 to 5 girl
It's not my place
In the 9 to 5 world