"Why would a dog slam her head off the floor? My thirteen year old lab has recently slamming her head off the floor."

This is the question I found in Yahoo Answers.

Why not use 'onto' here instead of 'off'?

As far as I know, 'off' is one of those 'source' prepositons: it shows an initial position when there is a change of location.

For example,

ex1) He jumped off the bridge.

ex2) He lifted it off the floor.

Wouldn't 'onto' make more sense than 'off' in the above sentence?
"Off" is wrong. 'Onto' is questionable. The expected is 'on'.
Mister Micawber, thanks for helping me out once again.Emotion: smile