In full view of others
I was in the college library sitting at one of those little cube desks wearing a T-shirt, shorts, and flip-flops. There were other cubes around and people scattered about, but nobody was in the cubes in the same line as me and therefore nobody was in the position to see my lower half.
I don't remember what I was reading but I was getting randy (probably nothing specific; I was always horny). I can often sit in such a way if my underwear and pants/shorts are just right so that I can rock my hips and my dick rubs up/down against the underwear or I can even put it up against the edge of the desk and rub it through my pants.
On this occasion I found a position that causing my growing hard-on to experience great pleasure rubbing against my boxers and the desk edge beyond. I was able to rock my lower body without moving the upper part of my body that others could easily see. I looked around and didn't notice any interest from others, so I continued.
. . . until several minutes later I finally unloaded into my boxers in the middle of the library. After a few minutes, the hard-on subsided and I gathered up my stuff and left. I went directly back to the dorms and cleaned up. What an awesome moment!
I still enjoy the technique and have been doing it while writing this. Time to finish.