Originally, the cow in a herd with a bell on its neck. The noise of the bell makes the herd easier to find.
Some people interpret the bell as signifying a leadership role, and its use reflects that misunderstanding.
"Dave, I need you to be the bell cow on this one."
In action... overheardActually, this definition sucks. Let me add my own. You can also share "bell cow" with a friend or share it on Twitter.