Mushmouth American slang
Someone who does not or is unable to express their feelings or ideas clearly.
Whenever I don't have enough sleep, I become a mushmouth.
I felt so ashamed of myself for being a mushmouth.
If I hadn't been a mushmouth, I would have been chosen for that position.
This slang dates from the mid 19th century.
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.