In American English there are many commonly used spoken language, although they are composed by simple words, but their meaning often puzzling people who want to learn English. There are some examples for your reference.
1. Head Start
If the two words ’beginning letters are written in capital, then it is a proper noun, referring to the U.S. government for the poor or handicapped children to establish a training institution, such institutions to help them before they enter a school able to catch up with the progress of teaching.
Many poor parents send their children to Head Start.
But if the two words are written as “head start”,it will be common nouns, meaning a head start over someone.
To know more colloquial expressions is a head start in learning English.
2. blue blood
It means rich or born of noble.
Some people do not want to admit that they have blue blood.
Many blue blooded children attend this private school.
3. bed of roses
It means bed of down; wonderful, ideal “a cosy nest”
Life is not always a bed of roses.
4. from soup to nuts
It refers to from start to finish, throughout, or goods are available in all varieties, intactness (mostly refers to commodities).
This store sells everything from soup to nuts.
But if it means knowledgeable, it would be another word of “from A to Z” instead of “from soup to nuts”.
Mr. Greenspan knows economics from A to Z.
“Off-limits”means no entry ;Restriction of passing in and out or not allowed touse, is close (closed or not available).
The library will be off-limits to the public until re-carpeting project is completed.
The teachers have the use of a john that is off-limits to students.
学生不得使用教师厕所。(john means toilet while it written in lowercase)
6. real bomb
In spoken language it refers to a failure in public places or on stage , or not good enough in performance.
The show was a real bomb, so I cut out early.