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1.
When having sex, move left to right, as opposed to up and down or forward and back, this is most easlily done by laying on a sleeping bag, or any other slidy surface.
Dude, when I was bangin' my girl, I used the z-axis and she orgasmed like MAD.
by Halo 2 Killa June 06, 2005
 
2.
a code used among teens to define a girl's butt in reference to the 3 dimensions on a graph. Z axis is how much it sticks out. Y axis is how tall it is. X axis is how wide it is.
That girl got mad z axis!
by SMDWCE3 January 28, 2009