1. supporting something.

While most of us might be OK about interracial dating, some people aren't on board with the notion.

2. working with someone.

It's a well-financed organization, and it has some well-known people on board.

Etymology: based on the literal meaning of on board (= on an aircraft, train, or ship)

• what will become of something
• What you don't know won't hurt .
• What you don't know won't hurt you.
• wheel and deal
• wheeler-dealer
• wheel has come full circle.
• wheel has turned full circle.
• wheeling and dealing
• wheels are turning
• when all is said and done
• when back is turned
• when hell freezes over
• When in Rome .
• When in Rome do as the Romans do.
• when it comes to the crunch
• when push comes to shove
• when ship comes in
• when someone's back is turned
• when someone sneezes, someone catches a cold
• when someone sneezes, something catches a cold
• when someone was a mere twinkle in father's eye
• when someone was a mere twinkle in their father's eye
• when someone was a twinkle in father's eye
• when someone was a twinkle in their father's eye
• when something sneezes, someone catches a cold