I guess this probably
originated from, of all places,
church. I suppose whenever a sermon got real good and over-zealous, someone might have said "
preach on!". (I could be
making that up, though.)
I generally say this whenever I think something someone is saying is
worthwhile and should be emphasized, continued, or something along those lines. Though I've cut down on my use of it, I say it rarely now, it used to provoke
curious glances and the like.
All in all, it's just another way of saying, "
hell yeah". (I wonder if that one originated in church..)