The Dinobots we can confirm as being built. And maybe it became a swearword BECAUSE it was the name of a Dinobot. Slag alone is like a destructive force of nature. So when they see him coming... "Oh, SLAG!"
Yes. Slag is basically melted metal, and is typically a waste product. Now, why do you think it makes sense for a race of robots to use it as a swear word, hmm? Also, it was the name of one of the original Dinobots.
I honestly thought that i misremembered that line but I was wrong. Hotshot actually did swear during this scene, and it aired on UK television unaltered?