When you think you are hidden behind an object but someone can clearly see the top of your head poking over it. The term is more widely used in video games where you are unaware of your players model and therefore have no idea your head is sticking out.
I saw that thundering bitch on the street giving out to a group of people from the tidy towns committee who advised her she has parked in a disabled spot, she's not disabled
A noun used to describe a person who rushes into abnormally stupid activities or situations he/she has no cause to be in or ability to successfully negotiate. i.e. one who "thunders" into stupid shit like a dumbass, hence the term. Usually reserved for those who are dumbasses to such a high degree / perform acts of dumbassery on such a regular basis, they require their own classification.
Al: "I'm going to go fight in a martial arts tournament on Tuesday."
Sal: "Dude, you've never taken martial arts before. You'll get your ass kicked. Why are you always looking for a fight?"
Al: "Nah man I'll fuck up anybody who tries that Judo shit on me"
Sal: "Stop being a thundering dumbass. Just have some drinks with the boys instead."
Al: *proceeds to get his ass kicked on Tuesday*