A stupid, incompetent person. Somebody who is totally uncomprehending and unaware of basic requirements in a range of social contexts. Also, somebody who is incapable of performing routine tasks.
Used in parts of Australia as a noun ("You're a bumper, mate!") and occasionally as an adjective, e.g "He supports a bumper football team."
"That bloke is useless, He's a total bumper!"
"He's too much of a bumper to ever work that out!"
"Only a bumper wouldn't be able to do that."
"That's a bumper thing to do!"