I also thought I invented this word but it seems there are plenty of other people out there with the same line of thinking.
The normal use of this word for me, however, is not to describe the steepness of a hill, but the steepness of a wave when surfing.
If a wave has enough stepth then it is steep enough to ride. Ususally used with the normal angle measurements like 90 degrees stepth etc.
"The surfs pretty full today, I think these waves need a bit more stepth."