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1.
A Tech-term usually used in the music-business to describe the MAX level a sound system can go to before distortion occurs:

More commonly known as "CLIPPING" ......i.e.
A squaring-off("clipped")waveform that occurs when the power output of an amplifier cannot meet the power requirement of the speaker system it is driving: Clipping can be very harmful to high-frequency speakers (tweeters).
"That sounds Crap, you've run out of Headroom, Turn it down or get a bigger Amp"
"Now the Dick-Heads blown his tweeters "
by Rod Attenborough October 13, 2006
 
2.
The amount of space available in a motor vechicle for a blow job.
Pete: Hey Jack nice truck.

Jack: Thanks for noticing. I had to get rid of the other one because of the lack of head room. The bench seat in this new one provides all the room Sheila needs.
by hardoneus maximus May 06, 2011