It means to join others in a project or agree with others about an idea of some sort.
Example:
"We're all going to begin recycling our plastic bags. We'd appreciate it if you'd get on board."
Originates from the idea of a group "boarding" a boat or ship. When you get onto a boat it is considered "getting on board the boat."
This is not an idiom. When you see AS ___ AS ___ you are looking at A Simile. This is comparing two flat things.
It means honest or out in the open. It comes from keeping your cards above the table to make it harder to cheat.
It's a boating phrase. Overboard means to go over the board, which is part of the boat. If you go overboard on a boat, you fall out into the water. As an idiom, it has come to mean doing so much that it seems excessive.
It's not really an idiom. It means "what are you thinking about."
RFP is not an idiom. It's an abbreviation.
It is not an idiom, it means your nose is itching.
The more common idiom is "stiff as a board," meaning "very stiff." Unless used ironically, like "straight as a crooked stick" or "straight as a politician's promise," the expression "straight as a board" would mean "very straight." When referring to "straight as..........." the term usually is as straight as a die
idiom means expression like a page in a book
It's not an idiom. It means the tip of your nostril.
"Sieve" is not an idiom. See the related link.
This is not an idiom. It is a measurement. $100,000 is how you write it in numbers.
Simply its mean a bully.