Lame-brain British derogatory slang informal
Used to refer to an idea that one foolishly utters or comes up with.
Don't bother Jackson. He spends most of his time thinking up lame-brain ideas.
Used to refer to a person who is stupid, foolish, or silly
You are such a lame-brain. How could you act like that in front him?
The plural form of this noun is "lame-brains".
The slang dates from 1921.
Children or adults often say this phrase to get their friends to join in something, especially to jump into a swimming pool (pond, lake, etc.)
“Last one in is a rotten egg!” yelled Jane, jumping into the pool.