Z-score 

The act of having sexual intercourse while in the same house, apartment, or general building of parents or family members.

'OMG, dude I totally pulled off a z-score last night with my gf. It was crazy! my heart was pounding.'
'Aw, man. I totally Z-scored ths moring, my parents were in the next room!!"

Z-score by Twisted Sisters April 29, 2008